Semiconductor device

ABSTRACT

A pin diode is formed by a p +  collector region, an n type buffer region, an n −  region and an n +  cathode region. A trench is formed from the surface of n +  cathode region through n +  cathode region to reach n −  region. An insulating film is formed along an inner wall surface of trench. A gate electrode layer is formed to oppose to the sidewall of n +  cathode region with insulating film interposed. A cathode electrode is formed to be electrically connected to n +  cathode region. An anode electrode is formed to be electrically connected to p +  collector region. The n +  cathode region is formed entirely over the surface between trenches extending parallel to each other. Thus, a power semiconductor device in which gate control circuit is simplified and which has good on property can be obtained.

This applicalion is a divisional of application Ser. No. 09/862,520,filed May 23, 2001 now U.S. Pat. No. 6,693,310, which is a divisional ofSer. No. 09/222,795, filed Dec. 30, 1998, now U.S. Pat. No. 6,265,735which is a divisional of application of Ser. No. 08/683,279, filed Jul.18, 1996, now U.S. Pat. No. 5,977,570.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vertical power semiconductor devicehaving self turn-off function and to a manufacturing method thereof.

2. Description of the Background Art

First, a conventional semiconductor device will be described.

FIG. 96 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with a first prior art example.Referring to FIG. 96, the first prior art example has an SITh (StaticInduction Thyristor). The SITh includes a pin diode porion, a p typegate region 307, a gate electrode layer 309, a cathode electrode 311 andan anode electrode 313.

A pin diode portion has a stacked structure including a p⁺, anode region301, an n⁻ region 303 and a cathode region (n⁺ emitter region) 305. Thep type gate region 307 is formed in n⁻ region 303. Gate electrode 309 iselectrically connected to p type gate region 307. Cathode electrode 311is electrically connected to cathode region 305, and anode electrode 313is electrically connected to p⁺ anode region 301, respectively.

The SITh can realize on-state by setting gate voltage applied to gateelectrode 309 positive. At this time, current flows through pin diodefrom p⁺ anode region 301 to the side of cathode region 305.

FIG. 97 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with a second prior art example.Referring to FIG. 97, the second prior art example shows a GTO (GateTurn-Off) thyristor. The GTO thyristor has a p⁺ anode region 351, an n⁻region 353, a p base region 355, a cathode region 357, a gate electrode359, a cathode electrode 361 and an anode electrode 363.

The p⁺ anode region 351, n⁻ region 353, p base region 355 and cathoderegion 357 are stacked successively. The p type base region 355 iselectrically connected to gate electrode 359. Cathode electrode 361 iselectrically connected to cathode region 357, and anode electrode 363 iselectrically connected to p⁺ anode region 351, respectively.

In this GTO thyristor also, on-state can be realized by setting the gatevoltage positive. By setting gate voltage positive, current flowsthrough a pnpn diode from p⁺ corrector region 351 to the side of cathoderegion 357.

Both in the first and second prior art examples, off-state can berealized by applying a negative voltage to the gate electrode. When anegative voltage is applied to gate electrode 309 or 359, minoritycarriers (holes) remaining in the device are extracted from gateelectrode 309 or 359. Thus, the main current is cut off.

FIG. 98 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with a third prior art example.Referring to FIG. 98, the third prior art example shows an example of atrench IGBT (Insulated Gate Bipolar Transistor). The trench IGBTincludes a p⁺ collector region 101, n⁺ buffer region 103, n⁻ region 105,p type base region 107, n⁺ emitter region 109, a p⁺ contact region 111,a gate oxide film 115, a gate electrode layer 117, a cathode electrode(emitter) 121 and an anode electrode (collector) 123. On p⁺ collectorregion 101, n⁻ region 105 is formed with n⁺ buffer region 103interposed. On n⁻ region 105, n⁺ emitter region 109 and p⁺ contactregion 111 are formed adjacent to each other with p type base region 107interposed. On the surface where n⁺ emitter region 109 is formed, thereis provided a trench 413.

Trench 413 passes through n⁺ emitter region 109 and p type base region107 and reaches n⁻ region 105. The depth T_(p) of trench 413 from thesurface is 3 to 5 μm.

Along inner wall surface of trench 413, gate oxide film 115 is formed.Gate electrode layer 117 is formed to fill the trench 413 and with itsupper end projecting from trench 413. Gate electrode layer 117 opposesto n⁺ emitter region 109, p type base region 107 and n⁻ region 105 withgate oxide film 115 interposed.

Interlayer insulating layer 119 is formed to cover an upper end of gateelectrode layer 117. In interlayer insulating layer, there is providedan opening which disposes the surfaces of n⁺ emitter region 109 and p⁺contact region 111. Cathode electrode (emitter) 121 is formed so as toelectrically connect n⁺ emitter region 109 and p⁺ contact region 111through the opening. Anode electrode (collector) 123 is formed to beelectrically connected to p⁺ collector region 101.

Hereinafter, the surface of the semiconductor substrate on which cathodeelectrode 121 is formed will be referred to as a cathode surface or afirst main surface, and the surface where anode electrode 123 is formedwill be referred to as an anode surface or the second main surface.

A trench MOS gate structure in which gate electrode layer 117 is formedin trench 413 with gate oxide film 115 interposed is manufacturedthrough the following steps.

First, in a semiconductor substrate, a relatively deep trench 413 ofabout 3 to about 5 μm is formed by common anisotropic dry etching.Sacrificial oxidation or cleaning is performed on the inner wall oftrench 413. Thereafter, a silicon thermal oxide film (hereinafterreferred to as a gate oxide film) 115 is formed at a temperature from900° C. to 1000° C. in, for example, vapor ambient (H₂O). A polysiliconfilm doped with an n type impurity such as phosphorous or apolycrystalline silicon film doped with a p type impurity such as boronfills the trench 413. The doped polysilicon film is patterned so thattrench 413 is filled and doped polysilicon film is drawn out at leastfrom a porion of trench 413 to the surface of the cathode side. Thepatterned doped polysilicon film is electrically connected to a gatesurface interconnection formed of a metal such as aluminum, providedentirely over the semiconductor device, while insulated from cathodeelectrode 121.

The method of controlling on-state and off-state in the third prior artexample will be described.

On-state is realized by applying a positive (+) voltage to gateelectrode 117 while a forward bias is applied between cathode electrode121-anode electrode 123, that is, while a positive (+) voltage isapplied to anode electrode 123 and a negative (−) voltage is applied tocathode electrode 121.

A turn-on process in which the device transits from off-state to theon-state will be described in the following.

When a positive (+) voltage is applied to gate electrode layer 117, an nchannel (inverted n region) which is inverted to n type and having veryhigh electron density is generated at p base region 107 near gate oxidefilm 115. Electrons, which are one of the current carriers (hereinafterreferred to as carriers) are injected from n⁺ emitter region 109 throughthe n channel to n⁻ region 105, and flow to p⁺ collector region 101 towhich the positive (+) voltage is applied. When the electrons reach p⁺collector region 101, holes, which are other current carrier areinjected from p⁺ collector region 101 to n⁻ region 105 and flow to n⁺emitter region 109 to which the negative (−) voltage is applied. Thus,the flow reaches the position where the aforementioned n channel is incontact with n⁻ region 105. This process is referred to as storageprocess, and the time necessary for this process is referred to asstorage time (t_(storage)) or turn-off delay time (td(_(off))). Powerloss during the storage time is so small that it can be neglected, ascompared with steady loss, which will be described layer.

Thereafter, from anode electrode 123 and cathode electrode 121,sufficient current carriers are stored in n⁻ region 105 to such anamount that is larger by two or three orders of magnitude than theconcentration of semiconductor substrate (1×10¹² to 1×10¹⁵ cm⁻³), inaccordance with the difference between potentials applied to bothelectrodes. Accordingly, a low resistance state referred to asconductivity modulation is caused by the hole-electron pairs, thusturn-on is completed. This process is referred to as a rise process, andthe time necessary for this process is referred to as rise time(t_(rise)). Power loss during this time is approximately the same orlarger than the steady loss, which will be described layer later, andconstitutes roughly one fourth of the entire loss.

The steady state after the completion of turn-on is referred to ason-state, and the power loss represented by a product of on-statevoltage caused by on resistance (effectively, potential differencebetween both electrodes) and the conduction current is referred to ason-loss or steady loss.

When a positive voltage is applied to gate electrode layer 117, an n⁺accumulation region 425 a having high electron density is formed alongthe sidewalls of trench 113, as shown in FIG. 99.

Off-state is realized by applying a negative (−) voltage to gateelectrode layer 117, even when forward bias is being applied to anodeelectrode 123-cathode electrode 121.

A turn off process in which the device transits from on state to offstate will be described in the following.

When a negative (−) voltage is applied to gate electrode layer 117, nchannel (inverted n region) formed on the side surface of gate electrodelayer 117 is eliminated, and supply of electrodes from n⁺ emitter region109 to n⁻ region 105 is stopped. The process up to here is referred toas storage process, and the time necessary for this process is referredto as storage time (ts) or turn off delay time (td(_(off))). The powerloss during this time is very small as compared with the turn on lossand the steady loss, and it can be neglected.

As the electron density reduces, the density of electrons which has beenintroduced to n⁻ region 105 gradually reduces from the vicinity of n⁺emitter region 109. In order to maintain charge neutralize condition,holes which have been introduced to n⁻ region 105 also reduce, and pbase region 107 and n⁻ region 105 are reversely biased. Consequently,depletion layer begins to extend at the interface between p base region107 and n⁻ region 105, and tends to have a thickness which correspondsto the applied voltage in the off state between both electrodes. Theprocess up to here is referred to as a fall process, and the timenecessary for this process is referred to as fall time (tf). The powerloss during this time is approximately the same or larger than theaforementioned turn off loss and steady loss, and it constitutes roughlyone fourth of the entire loss.

Further, holes in an electrically neutral region where both carriersremain outside the aforementioned depletion region (p⁺ collector region101) pass through the depletion region and extracted through p⁺ contactregion 111 to emitter electrode 121, thus carriers are all eliminatedand turn off is completed. This process is referred to tail process, andthe time necessary for this process is referred to as tail time(t_(tail)). The power loss during the tail time is referred to as tailloss, which is approximately the same or larger than the turn on loss,loss during the fall time and steady loss, and it constitutes roughlyone fourth of the entire loss.

The steady state after the completion of turn off is referred to as offstate and power loss caused by the product of leak current in this stateand the voltage between both electrodes is referred to as off loss.However, generally it is smaller than other power losses and it can beneglected.

The above described first and second prior art examples relate tocurrent control type devices in which minority carriers are extractedfrom gate electrodes 309 and 359 to set off-state. Therefore, at thetime of turn off, it is necessary to extract a considerable amount ofthe main current from the gate electrode. When a relatively largecurrent is to be extracted, there will be a large surge current causedby inductance of interconnections or the like, and heat radiation causedby current must also be taken into consideration. Therefore, it becomesnecessary to provide a protecting circuit against surge voltage andexcessive current, in the circuit for controlling the gate voltage. Thismakes the gate control circuit complicated. Further, it is possible thatthe control circuit is thermally destroyed or suffers from thermalrunaway because of heat, and hence a cooling mechanism must be provided.This makes the device larger.

A semiconductor device which solves these problems is disclosed inJapanese Patent Laying-Open No. 5-243561. The semiconductor devicedisclosed in this application will be described as a fourth prior artexample.

FIG. 100 is a plan view schematically showing the structure of thesemiconductor device in accordance with the fourth prior art example,and FIGS. 101 and 102 are cross sectional views taken along the linesP-P′ and Q-Q′ of FIG. 100, respectively.

Referring to FIGS. 100 to 102, the fourth prior art example shows anelectrostatic induction thyristor. On one surface of a high resistance ntype base layer 501, a p type emitter layer 503 is formed with an n typebuffer layer 502 interposed. On the other surface of n type base layer501, a plurality of trenches 505 are formed spaced by a small distancefrom each other. In these trenches 505, gate electrodes 507 are formedembedded, with gate oxide film 506 interposed. At every other regionbetween the trenches 505, n type turn off channel layer 508 is formed.On the surface of turn off channel layer 508, a p type drain layer 509is formed. At a surface portion sandwiched between p type drain layers509, an n type source layer 510 is formed.

A cathode electrode 511 is formed to be electrically connected to p typedrain layer 509 and n type source layer 510. An anode electrode 512 isformed to be electrically connected to p type emitter layer 503.

In the fourth prior art example, when the positive voltage is applied togate electrode 507 to raise the potential of n type base layer 501sandwiched between the trenches 505, electrons are introduced from ntype source layer 510, so that the device turns on. Meanwhile, when anegative voltage is applied to a gate electrode layer 507, a p typechannel is formed on a side surface of the trench of n type turn offchannel layer 508, carriers of n base layer 501 are discharged through pdrain layer 509 to cathode electrode 511, and therefore the device turnsoff.

In the fourth prior art example, the gate electrode 507 has an insulatedgate structure. Therefore, in the fourth prior art example, the gateelectrode 507 b is not of the current control type in which current isdirectly drawn out from the substrate, but it is of a voltage controlledtype in which control is realized by the voltage (gate voltage) appliedto the gate electrode.

Since the fourth prior art example is of the voltage controlled type, itis not necessary to extract a large current from gate electrode layer507 at the time of turn off. Accordingly, it is not necessary to providea protecting circuit or a cooling mechanism in consideration of surgecurrent and heat caused when large current is extracted. Therefore, thefourth prior art example is advantageous in that the gate controlcircuit can be simplified.

However, in the fourth prior art example, at the surface regionsandwiched between trenches 507 extending parallel to each other asshown in FIG. 100, there are p type drain layer 509 and n type sourcelayer 510 adjacent to each other. Since p type drain layer 509 has apotential barrier with respect to the electrons, the electron currententering the cathode electrode 511 flows only through the portion of ntype source layer 510. Therefore, there is inhibiting factor such aspartial increase in current density, which results in degraded oncharacteristics.

In the third prior art example shown in FIG. 98, it is not possible toimprove on-state voltage Vf, and hence power consumption of thesemiconductor device is considerably large. This will be described ingreater detail.

As a method of improving ON voltage (on-state voltage Vf of a diode)which is a basic characteristic of IGBT, there is a method of improvinginjection efficiency of electrons on the side of the cathode. In orderto improve injection efficiency of electrons, it is necessary toincrease impurity concentration on the side of the cathode or toincrease the effective cathode area. The effective cathode area meansthe area of a portion (denoted by the solid line in the figure) where n⁺region (effective cathode region) including n⁺ emitter region 109 andstorage region 425 a is in contact with p type base region 107 and n⁻region 105.

In the third prior art example, the depth of the trench 413 is 3-5 μm,as already described. Therefore, when a positive voltage is applied togate electrode layer, extension of the storage layer generated aroundthe trench 113 is limited. Accordingly, it is not possible to ensure thelarge effective cathode area. This hinders improvement in injectionefficiency of electrons on the side of the cathode, and hence ON voltageof IGBT cannot be reduced.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a power semiconductordevice which allows simplification of gate control circuit, providesgood on characteristic and reduces steady loss.

Another object of the present invention is to provide a powersemiconductor device which allows simplification of gate controlcircuit, has low on-state voltage Vf and low steady loss.

The semiconductor device in accordance with an aspect of the presentinvention including a diode structure in which main current flowsbetween both main surfaces sandwiching an intrinsic or a firstconductivity type semiconductor substrate includes a first impurityregion of a first conductivity type, a second impurity region of asecond conductivity type, a control electrode layer, a first electrodelayer and a second electrode layer. The first impurity region of thefirst conductivity type is formed on a first main surface of thesemiconductor substrate and has impurity concentration higher than thatof the semiconductor substrate. The second impurity region of the secondconductivity type is formed on a second main surface of thesemiconductor substrate, and sandwiches with the first impurity region,a low impurity concentration region of the semiconductor substrate. Thesemiconductor substrate has a plurality of trenches extending parallelto each other on the first main surface, and each trench is formed toreach the low impurity concentration region of the semiconductorsubstrate through the first impurity region from the first surface. Thefirst impurity region is formed entirely at the first main surface ofthe semiconductor substrate sandwiched by the trenches extendingparallel to each other. The control electrode layer is formed to opposeto the first impurity region and the low impurity concentration regionof the semiconductor substrate in the trench with an insulating filminterposed. The first electrode layer is formed on the first mainsurface of the semiconductor substrate and electrically connected to thefirst impurity region. The second electrode layer is formed on thesecond main surface of the semiconductor substrate and electricallyconnected to the second impurity region.

In the semiconductor device in accordance with one aspect of the presentinvention, the control electrode layer opposes to the first impurityregion and the low impurity concentration region of the semiconductorsubstrate with an insulating film interposed. In other words, the gatecontrol is of voltage control type. Therefore, it is not necessary toextract a large current from the control electrode at the time of turnoff. Therefore, it is not necessary to provide a protecting circuit fora cooling mechanism in the gate control circuit in consideration ofsurge voltage and heat caused when a large current flows. Therefore, ascompared with the first and second prior art examples, gate controlcircuit can be simplified.

Further, the device is a bipolar device. In the bipolar device, theholes and electrons contribute to the operation. Therefore, even whenthe substrate thickness is improved to meet the demand of higherbreakdown voltage and current path in the on state becomes longer,resistance can be maintained low, since there is generated conductivitymodulation by the holes and electrons. Therefore, power loss can bereduced and amount of heat radiation can be reduced.

Further, the control electrode layer opposes to the first impurityregion and a low impurity concentration region of the semiconductorsubstrate. Therefore, by applying a voltage to the control electrodelayer, the low impurity concentration region of the semiconductorsubstrate near the trench which is filled with the control electrodelayer can be turned to a channel having high electron densityapproximately the same as the density of first impurity region.Consequently, the channel region near the trench can be regarded as afirst impurity region, and hence a state as if the first impurity regionis enlarged can be realized. When the first impurity region is enlarged,the contact area between the low impurity concentration region of thesemiconductor substrate and the enlarged first impurity region, that is,the effective cathode area is increased. Thus, efficiency in injectingelectrons on the side of the cathode is improved, and on-state voltageVf of the diode can be reduced.

Further, only the first impurity region is formed on the first mainsurface of the semiconductor substrate sandwiched between the trenches.Therefore, as compared with an example in which impurity regions ofdifferent conductivity types exist on the first main surface, theelectron current entering from the cathode flows uniformly through thefirst main surface of the semiconductor substrate between the trenches.Accordingly, inhibiting factor such as partial increase in currentdensity can be eliminated, and good on characteristic is obtained.

In the above described aspect, preferably the plurality of trenchesinclude first, second and third trenches extending parallel to eachother. The first impurity region is formed entirely at the first mainsurface of the semiconductor substrate between the first and secondtrenches. A third impurity region of the second conductivity type isformed at the first main surface of the semiconductor substrate betweenthe second and third trenches. Therefore, the third impurity region isformed shallower than the trench, and is electrically connected to thefirst electrode layer.

At the first main surface of the semiconductor substrate, the thirdimpurity region is provided adjacent to the first impurity region with atrench interposed. The third impurity region has a conductivity typedifferent from that of the first impurity region. Therefore, at the timeof turn off of the device, holes are extracted from the third impurityregion. Thus, the speed of turn off of the device can be improved andthe turn off loss can be reduced.

The third impurity region is provided adjacent to the first impurityregion at the first main surface of the semiconductor substrate with atrench interposed. Therefore, by adjusting the ratio of existence of thethird and first impurity regions, desired turn off speed and on-statevoltage Vf can be selected.

According to another aspect of the present invention, the semiconductordevice includes a pnpn structure in which main current flows betweenboth main surfaces with an intrinsic or first conductivity typesemiconductor substrate sandwiched therebetween, which includes a firstimpurity region of a first conductivity type, a second impurity regionof a second conductivity type, a third impurity region of the secondconductivity type, a control electrode layer, a first electrode layerand a second electrode layer. The first impurity region of the firstconductivity type is formed at the first main surface of thesemiconductor substrate. The second impurity region of the secondconductivity type is formed at the second surface of the semiconductorsubstrate. The third impurity region of the second conductivity type isformed below the first impurity region to sandwich a region of thesemiconductor substrate with itself and the second impurity region. Thesemiconductor substrate has a plurality of trenches extending parallelto each other at the first main surface, and each trench is formed toreach a region of the semiconductor substrate through first and thirdimpurity regions from the first main surface. The first impurity regionis formed entirely at the first main surface of the semiconductorsubstrate sandwiched between the trenches extending parallel to eachother. The control electrode layer is formed to oppose to the first andthird impurity regions and the semiconductor substrate region with aninsulating film interposed, in the trench. The first electrode layer isformed on the first main surface of the semiconductor substrate andelectrically connected to the first impurity region. The secondelectrode layer is formed on the second main surface of thesemiconductor substrate and electrically connected to the secondimpurity region.

In the semiconductor device in accordance with aforementioned anotheraspect of the present invention, the control electrode layer opposes tothe first and third impurity regions and the semiconductor substrateregion with an insulating film interposed. In other words, the gatecontrol is of voltage controlled type. Therefore, it is not necessary toextract a large current from the control electrode layer at the time ofturn off. Accordingly, it is not necessary to provide a protectingcircuit or a cooling mechanism in the gate control circuit inconsideration of surge voltage or heat generated when a large currentflows. Therefore, compared with the first and second prior art examples,the gate control circuit can be simplified.

Further, the device is a bipolar device. In the bipolar device, bothholes and electrons contribute to the operation. Therefore, even whenthe substrate thickness is increased to meet the demand of higherbreakdown voltage and the current path in the on state becomes longer,there will be a conductivity modulation generated by the holes andelectrons. Therefore, the on resistance can be maintained low.Therefore, increase in steady loss can be suppressed and the amount ofheat radiation can be reduced.

Further, only the first impurity region is formed at the main surface ofthe semiconductor substrate between the trenches. Therefore, as comparedwith the examples in which impurity regions of different conductivitytypes exist at the first main surface, electron current entering fromthe cathode side flows uniformly through the first main surface of thesemiconductor substrate between the trenches. Therefore, inhibitingfactor such as partial increase in current density can be eliminated,and good on characteristic is obtained.

In the above described aspect, preferably, the plurality of trenchesinclude first, second and third trenches extending parallel to eachother. The first impurity region is formed entirely at the first mainsurface of the semiconductor substrate between the first and secondtrenches. A fourth impurity region of the second conductivity type isformed at the first main surface of the semiconductor substrate betweenthe second and third trenches. The fourth impurity region is madeshallower than the trench, and is electrically connected to the firstelectrode layer.

The fourth impurity region is provided at the first main surface of thesemiconductor substrate to be adjacent to the first impurity region withthe trench interposed. Further, the fourth impurity region has aconductivity type different from that of the first impurity region.Accordingly, holes are extracted from the fourth impurity region at thetime of turn off of the device. Therefore, turn off speed of the devicecan be improved and turn off loss can be reduced.

The fourth impurity region is provided adjacent to the first impurityregion with the trench interposed, at the first main surface of thesemiconductor substrate. Therefore, by adjusting the ratio of existenceof the fourth and first impurity regions, a desired turn off speed andon-state voltage can be selected.

In accordance with still further aspect of the present invention, thesemiconductor device includes a diode structure in which main currentflows between both main surfaces with an intrinsic or first conductivitytype semiconductor substrate sandwiched therebetween, which deviceincludes a first impurity region of a first conductivity type, a secondimpurity region of a second conductivity type, a third impurity regionof the second conductivity type, a fourth impurity region of the firstconductivity type, a control electrode layer, a first electrode layerand a second electrode layer. The first impurity region of the firstconductivity type is formed as the first main surface of thesemiconductor substrate, and has an impurity concentration higher thanthat of the semiconductor substrate. The second impurity region of thesecond conductivity type is formed on the second main surface of thesemiconductor substrate. The semiconductor substrate has trenchesextending parallel to each other and sandwiching the first impurityregion. The third impurity region of the second conductivity type is asidewall of the trench and formed at the first main surface. The fourthimpurity region of the first conductivity type is provided immediatelybelow the third impurity region to be in contact with the sidewall ofthe trench and the semiconductor substrate region, and has lowerconcentration than the first impurity region.

The control electrode layer is formed to oppose to the third and fourthimpurity regions and semiconductor substrate region with an insulatingfilm interposed, in the trench. The first electrode layer is formed onthe first main surface of the semiconductor substrate and iselectrically connected to the first and third impurity regions. Thesecond electrode layer is formed at the second main surface of thesemiconductor substrate and electrically connected to the secondimpurity region.

In the semiconductor device in accordance with aforementioned stillfurther aspect of the present invention, the control electrode layeropposes to the third and fourth impurity regions and the semiconductorsubstrate region with the insulating film interposed. In other words,the gate control is of voltage control type. Therefore, it is notnecessary to extract a large current from the control electrode layer atthe time of turn off. Therefore, it is not necessary to provide aprotecting circuit or a cooling mechanism in the gate control circuit inconsideration of surface voltage or heat radiation generated when alarge current flows. Therefore, as compared with the first and secondprior art examples, the gate control circuit can be simplified.

Further, the device is a bipolar device. In the bipolar device, both theholes and the electrons contribute to the operation. Therefore, even ifthe substrate thickness is increased to meet the demand of higherbreakdown voltage and current path in the on state becomes longer, therewill be conductivity modulation by the holes and electrons. Therefore,the resistance can be maintained low. Accordingly, the amount of heatradiation is small and increase in steady loss can be suppressed.

Further, the control electrode layer opposes to the third and fourthimpurity regions and the semiconductor substrate region. Therefore, byapplying a positive voltage to the control electrode layer, regions nearthe trenches in which control electrode layers are filled can have suchhigh electron density that is approximately the same as in the firstimpurity region. Therefore, all the regions near the trench can beregarded as the first impurity region, and a state as if the firstimpurity region is enlarged can be realized. When the first impurityregion is enlarged, the contact area between the enlarged first impurityregion and the semiconductor substrate region, that is, the effectivecathode area is increased. Thus, the efficiency in injecting electronson the side of the cathode is improved, and on-state voltage Vf of thediode can be reduced.

By applying a voltage to the control electrode layer, the region of theopposite conductivity type near the trench can have approximately thesame high electron density as that of the first impurity region.Therefore, the region of the opposite conductivity type such as thethird impurity region as well as the fourth impurity region can beregarded as the first impurity region. Since the third impurity regionis also regarded as a first impurity region in addition to the fourthimpurity region, the effective cathode area can further be increased.Thus, the efficiency in injecting electrons on the cathode side canfurther be improved, and the on-state voltage Vf on the diode canfurther be reduced.

Preferably, in the above described aspect, an isolating impurity regionis further provided, formed at the first main surface of thesemiconductor substrate. On one side of the outermost of the pluralityof trenches extending parallel to each other, another trench ispositioned, while on the other side, the isolating impurity region isformed in contact with the outermost trench and deeper than the trench.

Since isolating impurity region is provided to surround the region inwhich a diode structure or a thyristor structure is formed, the effectof electrical isolation from other elements can be enhanced, andbreakdown voltage of the device is improved and stabilized.

Preferably, in the above described aspect, the depth of the trench fromthe first main surface is at least 5 μm and at most 15 μm.

As the depth of the trench is at least 5 μm, the storage region havinghigh electron density can be generated widely along the sidewall of thetrench at on-state. Therefore, as compared with the third prior artexample, wider effective cathode area is ensured. Therefore, theefficiency in injecting electrons on the cathode side can further beimproved, and the on-state voltage Vf can be reduced. Further, since itis difficult to form a trench deeper than 15 μm with a minute width (ofat most 0.6 μm), the depth of the trench is at most 15 μm.

In the semiconductor device according to a still further aspect of thepresent invention, main current flows between both main surfaces of anintrinsic or a first conductivity type semiconductor substrate, and thedevice includes a first impurity region of a second conductivity type, asecond impurity region of a second conductivity type, a third impurityregion of the first conductivity type, a control electrode layer, andfirst and second electrode layers.

The first impurity region is formed on the side of the first mainsurface of the semiconductor substrate. The second impurity region isformed at the second main surface of the semiconductor substrate, andwith the first impurity region, sandwiches a low concentration region ofthe semiconductor substrate. The semiconductor substrate has a trenchreaching the semiconductor substrate region from the first main surfacethrough the first impurity region. The third impurity region is formedon the first impurity region to be in contact with the sidewall of thetrench of the first main surface of the semiconductor substrate. Thecontrol electrode layer is formed to oppose to the first and thirdimpurity regions and the semiconductor substrate region in the trenchwith an insulating film interposed, and controls current flowing betweenthe first and second main surfaces in accordance with an applied controlvoltage. The first electrode layer is formed on the first main surfaceof the semiconductor substrate and electrically connected to the firstand third impurity regions. The second electrode layer is formed on thesecond main surface of the semiconductor substrate and electricallyconnected to the second impurity region. When the first and second mainsurfaces of the semiconductor substrate is in a conducted state, anaccumulation region of the first conductivity type is formed around thetrench, to be in contact with the third impurity region. In theconduction state, the ratio Rn=(n/n+p) of the contact area n between theeffective cathode region including the third impurity region andaccumulation region with the first impurity region and the semiconductorsubstrate region with respect to the area p on the side of the firstmain surface of the first impurity region in at least 0.4 and at most1.0.

Since the ratio Rn is at least 0.4 and at most 1.0, which is higher thanthe third prior art example, efficiency in injecting electrons on theside of the cathode is improved as compared with a prior art example,and hence on-state voltage Vf can be reduced.

Preferably, in the above described aspects, the depth of the trench fromthe first main surface is at least 5 μm and at most 15 μm. Since thedepth of the trench is at least 5 μm, the storage region having highelectron density can be generated wider along the sidewall of the trenchat on-state. Therefore, wider effective cathode area than the thirdprior art example can be ensured. Therefore, the efficiency in injectingelectrons on the cathode side can further be enhanced, and on-statevoltage Vf can be reduced. In the present device, it is difficult toform a trench deeper than 15 μm with a minute width (of at most 0.6 μm),and hence the depth of the trench is at most 15 μm.

In the above described aspect, preferably, the trench includes aplurality of trenches, having first, second and third trenches. At thesemiconductor substrate between the first and second trenches, the firstand third impurity regions are formed. At the first main surface of thesemiconductor substrate between the second and third trenches, only thesemiconductor substrate region is positioned. On the semiconductorsubstrate between the second and third trenches, a conductive layer isformed with a second insulating layer interposed. The conductive layeris electrically connected to each of the control electrode layersfilling the second and third trenches.

Since the conductive layer is electrically connected to the controlelectrode layer, when a positive voltage, for example, is applied to thecontrol electrode layer at on-state, the positive voltage is alsoapplied to the conductive layer. The conductive layer opposes to thesemiconductor substrate region between the second and third trencheswith the second insulating layer interposed. Therefore, when thepositive voltage is applied to the conductive layer, the surface regionbetween the second and third trenches can have approximately the samehigh electron density as that of a third impurity region. Therefore, thethird impurity region is enlarged by the surface region of the substratesandwiched between the second and third trenches. Accordingly, theeffective cathode area is increase, efficiency in injecting electrons onthe cathode side can further be enhanced, and the on-state voltage Vf ofthe diode can further be reduced.

In the above described aspect, preferably, there are a plurality oftrenches, including first, second and third trenches. At thesemiconductor substrate between the first and second trenches, first andthird impurity regions are formed. At the first main surface of thesemiconductor substrate between the second and third trenches, thefourth impurity region of the second conductivity type having lowerconcentration than the second impurity region is formed. On thesemiconductor substrate between the second and third trenches, aconductive layer is formed with a second insulating layer interposed.The conductive layer is electrically connected to each of the controlelectrode layers filling the second and third trenches.

Since the conductive layer is electrically connected to the controlelectrode layer, when a positive voltage, for example, is applied to thecontrol electrode layer at on-state, the positive voltage is alsoapplied to the conductive layer. The conductive layer opposes to thefourth impurity region between the second and third trenches with thesecond insulating layer interposed. Since the fourth impurity region haslower concentration than the second impurity region, when the positivevoltage is applied to the conductive layer, the surface region betweenthe second and third trenches comes to have approximately the same highelectron density as that of the third impurity region. Therefore, thethird impurity region is enlarged by the surface area of the substratesandwiched between the second and third trenches. Thus, the effectivecathode area is increased, efficiency in injecting electrons on thecathode side is further enhanced, and the on-state voltage Vf diode canfurther be reduced.

Since the fourth impurity region is set to have lower concentration thanthe second impurity region, thyristor operation occurs when the deviceoperates. As a result, the ON voltage lowers advantageously when ratedcurrent is conducted.

When the device is turned off, a negative voltage, for example, isapplied to the control electrode layer. At this time, since the negativevoltage is also applied to the conductive layer, a region having higherhole density than the fourth impurity region is generated at the surfaceof the fourth impurity region below the conductive layer. Since theregion having a high hole density is formed, extraction of holes at thetime of turn off is facilitated, thus turn off speed of the device isimproved and the turn off loss can be reduced.

In the above described aspect, preferably, the fourth impurity region ofthe second conductivity type having lower concentration than the firstimpurity region is further provided to be in contact with the sidewallof the trench at a lower portion of the first impurity region and tosandwich with the second impurity region, the semiconductor substrateregion.

Since the fourth impurity region has lower concentration than the firstimpurity region, when a negative voltage is applied to the controlelectrode layer at off-state, there is generated a region having higherhole density than the concentration of the first impurity region, alongthe sidewall of the trench, in the fourth impurity region. Since theregion having high hole density is formed, extraction of holes, whichare carriers, can be facilitated and smoothly performed at the time ofturn off of the device, so that switching characteristic can beimproved.

In the semiconductor device in accordance with a still further aspect ofthe present invention, current flows between both main surfaces of anintrinsic or a first conductivity type semiconductor substrate, and thedevice includes a first impurity region of a second conductivity type, asecond impurity region of a second conductivity type, a third impurityregion of the first conductivity type, a fourth impurity region of thesecond conductivity type, a control electrode layer, and first andsecond electrode layers. The first impurity region is formed on the sideof the first main surface of the semiconductor substrate. The secondimpurity region is formed at the second main surface of thesemiconductor substrate and, sandwiches, with a first impurity region, alow concentration region of the semiconductor substrate. Thesemiconductor substrate has a trench reaching the semiconductorsubstrate region from the first main surface through the first impurityregion. The third impurity region is formed on the first impurity regionto be in contact with a sidewall of the trench at the first main surfaceof the semiconductor substrate. The fourth impurity region is formed tobe adjacent to the third impurity region at the main surface of thesemiconductor substrate on the first impurity region, and it has higherconcentration than the first impurity region.

The control electrode layer is formed to oppose to the first and thirdimpurity regions and the low concentration region of the semiconductorsubstrate with an insulating film interposed in the trench, and controlscurrent flowing between the first and second main surfaces in accordancewith an applied control voltage. The first electrode layer is formed atthe first main surface of the semiconductor substrate and electricallyconnected to the third and fourth impurity regions. The second electrodelayer is formed on the second main surface of the semiconductorsubstrate and electrically connected to the second impurity region.Here, the following relation holds where Dt represents the depth of thetrench from the first main surface, Wt represents the width of saidtrench, De represents the depth of the third impurity region from thefirst main surface, We represents the width of the third impurity regionfrom one trench to another trench, and Pt represents pitch betweenadjacent trenches:$\frac{{2\quad\left( {{We} + {Dt} - {De}} \right)} + {Wt}}{{2\left( {{We} + {Dt} - {De}} \right)} + {Pt}} \geqq 0.4$

The ratio Rn=(n/n+p) can be approximated as shown by the aboveexpression, in accordance with dimensions of various portions. Sincedimensions of various portions are set so that the ratio Rn is at least0.4, efficiency in injecting electrons on the side of the cathode can beimproved and the on-state voltage Vf can be reduced, as compared withthe third prior art example.

The method of manufacturing the semiconductor device in accordance witha present invention is for manufacturing a semiconductor device in whichmain current flows between both main surfaces of an intrinsic or a firstconductivity type semiconductor substrate, including the followingsteps.

First, by selective ion implantation to the first main surface of thesemiconductor substrate, a first impurity region of a secondconductivity type is formed. Then, the second impurity region of thesecond conductivity type is formed at the second main surface of thesemiconductor substrate. By selective ion implantation, a third impurityregion of the first conductivity type is formed at the first mainsurface in the first impurity region. By performing anisotropic etchingon the first main surface, a plurality of trenches including first,second and third trenches are formed at the semiconductor substrate.Thus, first and third impurity regions are formed along the sidewalls ofthe trench at the first main surface between the first and secondtrenches, and only a low concentration region of the semiconductorsubstrate is positioned at the first main surface between the second andthird trenches.

A control layer is formed in the trench to oppose to the lowconcentration region of the semiconductor substrate and the first andthird impurity regions between the first and second impurity regionswith an insulating film interposed. By selective ion implantation, aforth impurity region of a second conductivity type having higherimpurity concentration than the first impurity region is formed at thefirst main surface in the first impurity region, to be adjacent to thethird impurity region. A first electrode layer is formed on the firstmain surface to be electrically connected to the third and fourthimpurity regions. A second electrode layer is formed on the second mainsurface to be electrically connected to the second impurity region.

In accordance with a method of manufacturing a semiconductor device inaccordance with a present invention, only the low concentration regionof the semiconductor substrate is positioned at the first main surfacesandwiched between the second and third trenches. Therefore, the firstimpurity region is not positioned at the first main surface between thesecond and third trenches. Therefore, the object to improve devicecharacteristics by increasing the ratio Rn can be attained, and mainbreakdown voltage can be maintained.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing a structure of a semiconductordevice in accordance with Embodiment 1 of the present invention.

FIG. 2 is a schematic plan view showing the device of FIG. 1 withcathode electrode provided.

FIG. 3 is a schematic cross sectional view taken along the line A-A′ ofFIG. 2.

FIGS. 4 to 9 are schematic cross sectional views showing, in order, thesteps of manufacturing the semiconductor device in accordance withEmbodiment 1 of the present invention.

FIG. 10 is a schematic cross sectional view showing a main currentconducting state of the semiconductor device in accordance withEmbodiment 1 of the present invention.

FIG. 11 is a schematic plan view showing a structure of a semiconductordevice in accordance with Embodiment 2 of the present invention.

FIG. 12 is a schematic plan view showing the device of FIG. 11 withcathode electrode provided.

FIG. 13 is a schematic cross sectional view taken along the line B-B′ ofFIG. 12.

FIGS. 14 to 16 are schematic cross sectional views showing, in order,the steps of manufacturing the semiconductor device in accordance withEmbodiment 2 of the present invention.

FIG. 17 is a schematic plan view showing the structure of thesemiconductor device in accordance with Embodiment 3 of the presentinvention.

FIG. 18 is a schematic plan view showing the device of FIG. 17 withcathode electrode provided.

FIG. 19 is a schematic cross section taken along the line C-C′ of FIG.18.

FIGS. 20 and 21 are schematic cross sectional views showing, in order,the steps of manufacturing the semiconductor device in accordance withEmbodiment 3 of the present invention.

FIG. 22 is a graph showing relation between on-state voltage Vf and theratio Rn.

FIG. 23 is a schematic plan view showing a structure of thesemiconductor device in accordance with Embodiment 4 of the presentinvention.

FIG. 24 is a schematic plan view showing the device of FIG. 23 withcathode electrode provided.

FIG. 25 is a schematic cross sectional view taken along the line D-D′ ofFIG. 24.

FIG. 26 is a schematic plan view showing a structure of a semiconductordevice in accordance with Embodiment 5 of the present invention.

FIG. 27 is a schematic plan view showing the device of FIG. 26 withcathode electrode provided.

FIG. 28 is a schematic cross sectional view taken along the line E-E′ ofFIG. 27.

FIGS. 29 and 30 are schematic cross sectional views showing, in order,the steps of manufacturing the semiconductor device in accordance withEmbodiment 5 of the present invention.

FIG. 31 is a schematic plan view showing the structure of thesemiconductor device in accordance with Embodiment 6 of the presentinvention.

FIG. 32 is a schematic plan view showing the device of FIG. 31 withcathode electrode provided.

FIG. 33 is a schematic cross section taken along the line F-F′ of FIG.32.

FIG. 34 is a plan view schematically showing the structure of thesemiconductor device in accordance with Embodiment 7 of the presentinvention.

FIG. 35 is a schematic plan view showing the device of FIG. 34 withcathode electrode provided.

FIG. 36 is a schematic cross sectional view taken along the line G-G′ ofFIG. 35.

FIGS. 37 and 38 are schematic cross sectional views showing, in order,the steps of manufacturing the semiconductor device in accordance withEmbodiment 7 of the present invention.

FIG. 39 is a plan view schematically showing a structure of asemiconductor device in accordance with Embodiment 8 of the presentinvention.

FIG. 40 is a schematic plan view showing the device of FIG. 39 withcathode electrode provided.

FIG. 41 is a schematic cross sectional view taken along the line H-H′ ofFIG. 40.

FIG. 42 is a schematic plan view showing the structure of asemiconductor device in accordance with Embodiment 9 of the presentinvention.

FIG. 43 is a schematic plan view showing the device of FIG. 42 withcathode electrode provided.

FIG. 44 is a schematic cross sectional view taken along the line I-I′ ofFIG. 43.

FIGS. 45 to 48 are schematic cross sectional views showing, in order,the steps of manufacturing the semiconductor device in accordance withEmbodiment 9 of the present invention.

FIG. 49 is a schematic cross sectional view showing a main currentconducting state of the semiconductor device in accordance withEmbodiment 9 of the present invention.

FIG. 50 is a plan view schematically showing a structure of asemiconductor device in accordance with Embodiment 10 of the presentinvention.

FIG. 51 is a schematic plan view showing the device of FIG. 50 withcathode electrode provided.

FIG. 52 is a schematic cross sectional view taken along the line K-K′ ofFIG. 51.

FIG. 53 is a schematic cross sectional view showing the method ofmanufacturing the semiconductor device in accordance with Embodiment 10of the present invention.

FIG. 54 is a schematic plan view showing trenches arrangedconcentrically.

FIG. 55 is a schematic plan view showing trenches arrangedconcentrically.

FIG. 56 is a schematic plan view showing trenches arrangedconcentrically.

FIG. 57 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with Embodiment 11 of the presentinvention.

FIGS. 58 to 62 are schematic cross sectional views showing, in order,the method of manufacturing the semiconductor device in accordance withEmbodiment 11 of the present invention.

FIG. 63 is a cross sectional view schematically showing the structure ofthe semiconductor device in accordance with Embodiment 12 of the presentinvention.

FIGS. 64 to 67 are schematic cross sectional views showing, in order,the method of manufacturing a semiconductor device in accordance withEmbodiment 12 of the present invention.

FIG. 68 is a schematic cross sectional view showing on-state of thesemiconductor device in accordance with Embodiment 12 of the presentinvention.

FIG. 69 is a cross sectional view schematically showing a structure of athe semiconductor device in accordance with Embodiment 13 of the presentinvention.

FIG. 70 shows a step of manufacturing the semiconductor device inaccordance with Embodiment 13 of the present invention.

FIG. 71 is a schematic cross sectional view showing on-state of thesemiconductor device in accordance with Embodiment 13 of the presentinvention.

FIG. 72 is a cross sectional view schematically showing the structure ofa semiconductor device in accordance with Embodiment 14 of the presentinvention.

FIG. 73 is a partial cross sectional view schematically showing astructure of a semiconductor device in accordance with Embodiment 15 ofthe present invention.

FIG. 74 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with Embodiment 16 of the presentinvention.

FIGS. 75 to 84 are schematic cross sectional views showing, in order,the steps of manufacturing the semiconductor device in accordance withEmbodiment 17 of the present invention.

FIGS. 85 and 86 show manufacturing steps when p type base region isprotruded.

FIGS. 87 and 88 show manufacturing steps when p type base region issmall.

FIG. 89 shows a manufacturing step showing isotropic dry etchingperformed after the formation of a trench.

FIG. 90 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with Embodiment 18 of the presentinvention.

FIG. 91 shows a step of manufacturing the semiconductor device inaccordance with Embodiment 18 of the present invention.

FIG. 92 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with Embodiment 19 of the presentinvention.

FIG. 93 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with Embodiment 20 of the presentinvention.

FIG. 94 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with Embodiment 21 of the presentinvention.

FIG. 95 is a cross sectional view schematically showing a structure of asemiconductor device in accordance with Embodiment 22 of the presentinvention.

FIG. 96 is a schematic cross sectional view showing a structure of asemiconductor device in accordance with a first prior art example.

FIG. 97 is a schematic cross sectional view showing a structure of asemiconductor device in accordance with a second prior art example.

FIG. 98 is a schematic cross sectional view showing a structure of asemiconductor device in accordance with a third prior art example.

FIG. 99 is a schematic cross sectional view showing how an n⁺accumulation layer is generated in the third prior art example.

FIG. 100 is a plan view schematically showing a structure of asemiconductor device in accordance with a fourth prior art example.

FIG. 101 is a schematic cross sectional view taken along the line P-P′of FIG. 100.

FIG. 102 is a schematic cross sectional view taken along the line Q-Q′of FIG. 100.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described with reference tothe figures.

In the following, for convenience, the cathode region, which is an n⁺high concentration impurity region will be sometimes referred to as n⁺emitter region, and the anode region which is p⁺ high concentrationimpurity region will be sometimes referred to as p⁺ collector region.

[Embodiment 1]

Referring to FIGS. 1 to 3, the present embodiment shows an example whichincludes a pin diode. The pin diode includes a p⁺ anode (collector)region 1 of a second conductivity type formed at a second main surface,an n type buffer region 3, an n⁻ region 5 which is the semiconductorsubstrate of a first conductivity type of low impurity concentration. Ann⁺ cathode region (n⁺ emitter region) 7 of the first conductivity typeformed at the first main surface, insulating films 11 and 15, a gateelectrode layer 13 which is the control electrode layer, a cathodeelectrode 17 which is a first electrode layer, and an anode electrode 19which is a second electrode layer.

At the first main surface where the cathode region 7 is provided, thereis formed a trench 9, which reaches the n⁻ region 5 of the substratethrough n⁺ cathode region 7.

As shown in FIG. 1, the trench 9 has such planer shape thatapproximately surrounds a rectangle, and has portions extending parallelto each other within the rectangle.

The n⁺ cathode region 7 is formed entirely at the first main surface ofthe semiconductor substrate sandwiched between trenches 9 extending inparallel.

The width W of trench 9 is, for example, at least 0.8 μm and at most 1.2μm. The depth D₁ is, practically, from 5.0 μm to 15.0 μm.

Along the inner wall surface of trench 9, a gate insulating film 11 (forexample, a silicon thermal oxide film) is provided. Gate electrode layer13 is formed of phosphorus doped polycrystalline silicon film to fillthe trench 9, with its upper end projecting from first main surface.

Gate electrode layer 13 opposes to the side surface of n⁺ cathode region7 and to the side surface and bottom surface of n⁻ region 5, with gateinsulating film 11 interposed.

Gate electrode layer 13 may be pulled up to a portion where insulatingfilm is provided on the first main surface, from the trench (not shown).

An insulating film 15 of, for example, a silicon thermal oxide film andBPSG (Boro Phospho-Silicate Glass) is formed to cover the upper end ofthe gate electrode layer 13.

There is provided an opening at a portion of the BPSG insulating film15, and metal interconnection is connected to the gate electrode throughthe opening (not shown).

The cathode electrode 17, which is the first electrode layer, iselectrically connected to cathode region 7. Cathode region 7 is formedon a region surrounded by trenches 9. The planer region where thecathode electrode 17 is formed is referred to as a diode forming regionhere.

Anode electrode 19, which is the second electrode layer, is electricallyconnected to p⁺ collector region 1 formed at the second main surface.

As to the impurity concentrations of respective portions, the surfaceconcentration of p⁺ collector region 1 is from 1×10¹⁶ cm⁻³ to 5×10²¹cm⁻³, the peak concentration of n type buffer region 3, 1×10¹³ cm⁻³ to1×10¹⁹ cm⁻³, of n⁻ region 5, 1×10¹² cm⁻³ to 1×10¹⁷ cm⁻³, and of cathoderegion 7, the,surface concentration must be at least 1×10¹⁷ cm⁻³. Thesurface impurity concentration of p⁺ collector region is higher than thepeak impurity concentration of n type buffer region 3, the peak impurityconcentration of n type buffer region 3 is higher than peak impurityconcentration of n⁻ region 5, and the surface impurity concentration ofn⁺ cathode region 7 is higher than the peak concentration of n⁻ region5.

The impurity concentration of n type buffer region 3 has only to belower than the impurity concentration of p⁺ collector region 1 andhigher than that of n⁻ region 5.

The method of manufacturing the semiconductor device in accordance withthe present embodiment will be described.

First, referring to FIG. 4, p⁺ collector region 1, n type buffer region3 and n⁻ region 5 are formed stacked in this order.

Referring to FIG. 5, trenches 9 a are selectively formed, for example,by photolithography and anisotropic plasma etching used in a commonsemiconductor process, to extend from the surface of n⁻ region 5 to theinside.

Referring to FIG. 6, an insulating film 11 of, for example, siliconoxide film, which will be the gate insulating film, is formed along theinner wall surface of trench 9 by, for example, thermal oxidation.

Before formation of gate oxide film 11, sacrificial oxidation andisotropic plasma etching (CDE) may be performed to improve MOScharacteristic.

Referring to FIG. 7, gate electrode layer 13 is formed to fill thetrench 9 with its upper end protruding from trench 9 by commonphotolithography and etching technique. Gate electrode layer 13 isformed of a material such as polycrystalline silicon doped with an ntype impurity such as phosphorus (hereinafter referred to as dopedpolysilicon).

Referring to FIG. 8, an insulating film 15 formed of a silicon thermaloxide film and a CVD oxide film such as BPSG is formed to cover theupper end of gate electrode layer 13 protruding from trench 9.

Referring to FIG. 9, thereafter, selective ion implantation of n typeimpurity element such as Sb, As, P or the like is performed to thesurface of n⁻ region 5 sandwiched between the trenches 9. Thereafter,introduced impurity is diffused by heat treatment, for example, and n⁺cathode region 7 is formed entirely at the surface of n⁻ region 5sandwiched between the trenches. The cathode region 7 is made shallowerthan the depth of trench 9.

Thereafter, the cathode electrode 17 is formed to be electricallyconnected to cathode region 7, and anode electrode 19 is formed to beelectrically connected to p⁺ collector region 1. Thus, the semiconductordevice shown in FIGS. 2 and 3 is completed.

The method of operating the semiconductor device in accordance with apresent embodiment will be described.

Referring to FIG. 3, on-state is realized by applying a small positivevoltage to the gate electrode layer 13. In this case, current flows fromp⁺ collector region 1 to n⁺ cathode region 7. This operation is the sameas the pin diode, so that electrons are introduced from n⁺ cathoderegion 7 to n⁻ semiconductor substrate 5, holes are introduced from p⁻collector region 1, whereby conductivity modulation occurs in n⁻substrate 5. Thus, on-state voltage becomes lower.

Off-state is realized by applying a negative voltage to gate electrodelayer 13. When a negative voltage is applied to gate electrode layer 13,a depletion layer extends around the trench 9, the current path of themain current is cut off, and thus the device can be turned off.

In the semiconductor device in accordance with the present embodiment,gate electrode layer 13 opposes to n⁻ region 5 and the sidewall ofcathode region 7 with insulating film 11 interposed, as specificallyshown in FIG. 3. In other words, the control method by the gateelectrode layer 13 is of voltage controlled type. Therefore, differentfrom SITh in which gate is formed by pn junction, gate electrode layer13 never extracts a part of the main current as gate current in the turnoff operation. Therefore, it is not necessary to provide large currentto the gate control circuit. Therefore, the gate drive circuit can besimplified, it is not necessary to provide a protecting circuit inconsideration of a surge current which is generated when a gate currentflows, and a cooling apparatus in consideration of heat radiation is notnecessary, either. Therefore, as compared with the first and secondprior art examples, gate control circuit can be simplified in thesemiconductor device in accordance with the present embodiment, and thesystem as a whole can be reduced in size, simplified and enables lowenergy loss.

Further, the pin diode is a bipolar device. In the bipolar device, bothholes and electrons contribute to the operation. Therefore, even if thesubstrate thickness is increased to meet the demand of higher breakdownvoltage, especially the thickness T₀ of n⁻ region 5 of FIG. 3 isincreased and current path when the pin diode operates becomes longer,there will be conductivity modulation generated by holes and electrons.Therefore, on-state voltage can be maintained low. Therefore, increasein steady loss can be suppressed and the amount of heat radiation can bereduced.

Further, as shown in FIG. 3, gate electrode layer 13 opposes to n⁻region 5 and cathode region 7, as shown in FIG. 3. Therefore, when apositive voltage is applied to gate electrode layer 13 at on-state,there is generated an n⁺ accumulation region 21 where the large numberof electrons are pulled, around the trench 9, as shown in FIG. 10.Therefore, n⁺ region serving as cathode region 7 is enlarged.

Now, as a method of improving on-state voltage Vf of the diode, it hasbeen known to increase effective cathode area, as already described. Theeffective area of cathode here means the interface area between n⁻region and n⁺ region, and n⁺ region connected with cathode electrode.

In the semiconductor device in accordance with the present embodiment,since there is generated n⁺ accumulation region 21 as shown in FIG. 10,n⁺ cathode region 7 is enlarged. Therefore, the contact area between theentire effective cathode region including n⁺ cathode region 7 plus n⁺accumulation region 21 and n⁻ region 5 is enlarged. Thus, injectionefficiency of electron on the side of the cathode can be improved, andon-state voltage Vf of the diode can be reduced. In this manner, evenwhen the first main surface (cathode side) is the n⁺ cathode region inits entirety, it becomes possible to reduce on loss by increasing n⁺region in the semiconductor chip as a whole, by enlarging effectivecathode region. In other words, power loss of the semiconductor devicecan be reduced. In the semiconductor device in accordance with thepresent embodiment, since n⁺ cathode region 7 is formed entirely at thefirst main surface on the cathode side, as compared with the example inwhich n region and p region exist at the first main surface (FIGS. 100to 102), the electron current entering from the cathode side flowsuniformly through the first main surface of the semiconductor devicesandwiched by the trenches 9. Therefore, partial increase of currentdensity can be prevented, and good on characteristic is obtained.

[Embodiment 2]

Referring to FIGS. 11 to 13, as compared with the semiconductor devicein accordance with Embodiment 1, the semiconductor device of the presentembodiment is different in that a p⁺ isolation impurity region 23 isprovided.

The p⁺ isolation impurity region 23 is formed at the surface of n⁻region 5 to surround planer region of diode forming region and to be incontact with trenches 9. Further, p⁻ isolation impurity region 23 ismade deeper than trench 9.

Except this point, the present embodiment is the same as Embodiment 1.Therefore, corresponding portions are denoted by the same referencecharacters and description thereof is not repeated.

The method of manufacturing the semiconductor device in accordance withthe present embodiment will be described.

The method of manufacturing the semiconductor device in accordance withthe present embodiment first includes the same steps as Embodiment 1shown in FIG. 4. Thereafter, referring to FIG. 14, a p⁺ region 23 a isformed selectively at a position surrounding the diode forming region,by deposition or ion implantation of an element such as B, serving as ptype impurity. Thereafter, heat treatment or the like is performed.

Referring to FIG. 15, by the above described heat treatment, p typeimpurity is diffused, and p⁺ isolation impurity region 23 is formed at aprescribed position.

Referring to FIG. 16, thereafter, a trench 9 a is formed having portionsextending parallel to each other at the surface of n⁻ region 5.Thereafter, approximately similar processes as in Embodiment 1 areperformed. Therefore, the description thereof is not repeated.

The method of operating the semiconductor device by the gate isapproximately the same as Embodiment 1.

Referring to FIG. 13, the p⁺ isolation impurity region 23 is connectedto cathode electrode 17 by an inverted layer formed around gateelectrode layer 13, when a negative voltage is applied to gate electrodelayer 13. Therefore, the pn junction formed by p⁺ isolation impurityregion 23 and n⁻ region 5 is reversely biased. Therefore, main breakdownvoltage maintaining capability of the device can be enhanced.

According to a semiconductor device of the present embodiment, p typeimpurity region 23 is formed deeper than trench 9 to surround diodeforming region, as shown in FIGS. 12 and 13. Therefore, p⁺ isolatedregion 23 is electrically isolated from the diode at on-state, on-statevoltage can be maintained low. And, p⁺ isolated region 23 iselectrically connected cathode electrode 17 at off-state, breakdownvoltage can be improved.

[Embodiment 3]

Referring to FIGS. 17 to 19, the semiconductor device of the presentembodiment differs from the semiconductor device of Embodiment 1 in thata p⁺ high concentration region 31 (hereinafter referred to as p⁺ contactregion) is provided.

The p⁺ contact region 31 is formed at the first main surface in diodeforming region, to be adjacent to n⁺ cathode region with trenches 9 band 9 c interposed. The p⁺ contact region 31 is formed at the surfaceregion sandwiched by trenches 9 b and 9 c extending parallel to eachother, as shown in FIG. 18. The p⁺ contact region 31 is electricallyconnected to cathode electrode 17. The p⁺ contact region 31 has asurface impurity concentration of at least 1×10¹⁷ cm⁻³. The p⁺ contactregion 31 and the n⁺ cathode region 7 are arranged alternately, withtrenches interposed. The number of trenches 9 a and 9 b, . . . may bearbitrarily selected.

Except this point, the present embodiment is almost similar toEmbodiment 1. Therefore, corresponding portions are denoted by the samereference characters and description thereof is not repeated.

The method of manufacturing the semiconductor device in accordance withthe present embodiment will be described.

One of the method of manufacturing the semiconductor device inaccordance with the present embodiment includes similar steps asEmbodiment 1 shown in FIGS. 4 to 8. Then, referring to FIG. 20, bycommon photolithography process, portions other than the portion wherep⁺ contact region is to be formed are masked by photoresist, and by ionimplantation, depletion or the like of element such as boron serving asp type impurity, a p⁺ contact region 31 is formed at the surface of n⁻region 5 sandwiched between trenches 9 b, 9 c and so on extendingparallel to each other. The p⁺ contact region 31 has the depth of about0.5 μm to about 1.0 μm, and is made shallower than trench 9.

Again, referring to FIG. 21, by the combination of photolithographyprocess and ion implantation process similar to those described above,n⁺ cathode region 7 is formed entirely at the surface of n⁻ region 5sandwiched between trenches 9 a and 9 b, and 9 c and 9 d, to be adjacentto p⁺ contact region 31 with trench 9 b or 9 c interposed. The followingsteps are approximately similar to those of Embodiment 1, and therefore,description thereof is not repeated.

The order of forming p⁺ contact region 31 and n⁺ cathode region 7 may bereversed. Elements and heat treatment used for diffusion of respectiveregions may be adjusted in accordance with the desired depth ofdiffusion.

The method of operating the semiconductor device in the presentembodiment is also the same as Embodiment 1. Therefore, descriptionthereof is not repeated.

In the semiconductor device in accordance with the present embodiment,p⁺ contact region 31 is arranged to be adjacent to n⁺ cathode region 7with trench 9 b or 9 c interposed, as shown in FIG. 19. Therefore,on-state voltage Vf can be reduced, and turn-off time can be reduced.These points will be described in greater detail in the following.

FIG. 22 is a graph showing relation between on-state voltage Vf andratio Rn, which is obtained by a simulation of a general trench IGBT ora trench diode. The ratio Rn here means the ratio of existence of n typeimpurity region when there are n type impurity region 7 and p typeimpurity region 31 on the side of the first main surface (cathode side)as shown in FIGS. 18 and 19, which is obtained in accordance with thefollowing equation.

Here, the effective cathode region includes n⁺ accumulation region 21(FIG. 10) provided when a positive voltage is applied to the gateelectrode.Rn=n ⁺ region (effective cathode region)/(n ⁺ region (effective cathoderegion)+p type region)  (1)

As is apparent from FIG. 22, the larger the ratio Rn, that is, thelarger the ratio of existence of n type impurity region, the lower theon-state voltage Vf. Accordingly, on-state voltage can be minimized,when there is no p type impurity region (that is, when the ratio Rn=1).

As shown in FIG. 19, in the semiconductor device in accordance with thepresent embodiment, p⁺ contact region 31 is provided adjacent to n⁺cathode region 7. Therefore, hole current I₁ is drawn out from p⁺contact region 31 to cathode electrode 17. Hole current I₁ is a part ofthe total hole current at turn-off. Therefore, current I flowing thoughthe diode is reduced, and especially the tail current tends to decreasequickly. Thus, the turn off time can be reduced.

Accordingly, in the semiconductor device in accordance with the presentembodiment, by adjusting the ratio of existence of cathode region 7 andp⁺ contact region 31 at the surface of n⁻ region 5, optimum on-statevoltage Vf and turn-off time in accordance with various diode propertiescan be selected in accordance with the expression (1) above.

[Embodiment 4]

Referring to FIGS. 23 to 25, the semiconductor device in accordance withthe present embodiment differs from Embodiment 3 in that a p⁺ isolationimpurity region 23 is provided.

The p⁺ isolation impurity region 23 is formed at the surface of n⁻region 5 to surround the planer region of the diode forming region andto be in contact with trench 9. The p⁺ isolation impurity region 23 ismade deeper than the trench 9.

Except these points, the present embodiment is the same as Embodiment 3.Therefore, corresponding portions are denoted by the same referencecharacters and description thereof is not repeated.

Referring to FIG. 25, the p⁺ isolation impurity region 23 is connectedto cathode electrode 17 by an inverted layer formed around gateelectrode layer 13, when a negative voltage is applied to gate electrodelayer 13. Therefore, the pn junction formed by p⁺ isolation impurityregion 23 and n⁻ region 5 is reversely biased. Therefore, main breakdownvoltage maintaining capability of the device can be enhanced.

According to a semiconductor device of the present embodiment, p typeimpurity region 23 is formed deeper than trench 9 to surround diodeforming region, as shown in FIGS. 24 and 25. Therefore, p⁺ isolatedregion 23 is electrically isolated from the diode at on-state, on-statevoltage can be maintained low. And, p⁺ isolated region 23 iselectrically connected cathode electrode 17 at off-state, breakdownvoltage can be improved.

[Embodiment 5]

Referring to FIGS. 26 to 28, the present embodiment shows an examplewhich has four layered pnpn thyristor. The four layered pnpn diodeincludes p⁺ collector region 1, an n type buffer region 3, an n⁻ region5, a p type base region 41 and an n⁺ cathode region 7. These p⁺collector region 1, n type buffer region 3, n⁻ region 5, p type baseregion 41 and n⁺ cathode region 7 are stacked successively. From thesurface of n⁺ cathode region 7, a trench 9 is formed to reach n⁻ region5 through n⁺ cathode region 7 and p type base region 41, and to haveportions extending parallel to each other. The n⁺ cathode region 7 isformed entirely at the surface sandwiched between the trench 9 extendingparallel to each other.

The p type base region 41 has peak impurity concentration of from 1×10¹⁴cm⁻³ to 5×10¹⁷ cm⁻³, and the n⁺ cathode region 7 has a surface impurityconcentration of at least 1×10¹⁷ cm⁻³. The surface impurityconcentration of n⁺ cathode region 7 is higher than the peak impurityconcentration of p type base region 41.

Other structures are the same as those of Embodiment 1. Therefore,corresponding portions are denoted by the same reference characters anddescription thereof is not repeated.

The method of manufacturing the semiconductor device in accordance withthe present embodiment will be described.

First, the method of manufacturing of the present embodiment includesthe same steps as those of Embodiment 1 shown in FIGS. 4 to 8.Thereafter, referring to FIG. 29, p type base region 41 is formed at aportion of the first main surface of n⁻ region 5 sandwiched by paralleltrenches 9 by ion implantation and diffusion, for example. The p typebase region 41 is formed such that it has peak impurity concentration of1×10¹⁴ cm⁻³ to 5×10¹⁷ cm⁻³, and that is shallower than trench 9 anddeeper than n⁺ cathode region 7, which will be described later. That is,it is formed to have the depth of 1.0 μm to 15.0 μm, for example.

Referring to FIG. 30, n⁺ cathode region 7 is formed by ion implantationand diffusion, for example, at the first main surface sandwiched bytrenches 9 extending parallel to each other. The n⁺ cathode region 7 isformed such that it has surface impurity concentration of at least1×10¹⁸ cm⁻³ and that it is shallower than p type base region 41.Subsequent steps are the same as those of Embodiment 1. Therefore,description thereof is not repeated.

The method of operating the semiconductor device in accordance with thepresent embodiment will be described.

On-state is realized by applying a positive voltage to gate electrodelayer 13 shown in FIG. 28. When a positive voltage is applied to gateelectrode layer 13, the portion of p type base region 41 which opposesto gate electrode layer 13 is inverted to n⁺ region, thus providing achannel and electron current flows. Then, corresponding to the electroncurrent, holes are introduced from p⁺ anode region 1 to n⁻ semiconductorsubstrate 5, causing conductivity modulation. Further, the hole currenteventually enters p base region 41. When this current increases, thepotential and p type base region 41 increases, and if the potentialbecomes larger than the internal potential, the diode provided by p typebase region 41 and n⁺ cathode region 7 becomes turned on. Thus, currentflows from n⁺ cathode region 7 through p base region 41 directly to n⁻semiconductor substrate 5. Thus the four layered pnpn thyristor turnson, realizing on-state of the present embodiment.

Off-state is realized by applying a negative voltage to gate electrodelayer 13 shown in FIG. 28. When a negative voltage is applied to gateelectrode layer 13, n⁺ channel, which was formed in the on state, iseliminated, supply of electrons from n⁺ cathode region is stopped, andsimultaneously, depletion layer extends from gate electrode layer 13 ton⁻ region 5. Thus, current path is pinched off, and current is reduced.And the device is turn off, when current is smaller than holding currentof the thyristor provided by n⁺ cathode 7, p type base region 41, n⁻region 5, and p⁺ anode region 1.

After the main current is cut off, the same breakdown voltage ismaintained by the above described p type base region 41. Therefore, inthe present embodiment, it is hot necessary to apply a gate voltage tomaintain off-state.

In the present embodiment, gate electrode layer 13 opposes to n⁻ region5, p type base region 41 and cathode region 7 with insulating layer 11interposed as shown in FIG. 28. In other words, the gate control methodis of voltage controlled type. Therefore, as already described withreference to Embodiment 1, the gate control circuit can be simplified ascompared with the current controlled type device. Further, cathoderegion 7 having a large area is formed at the first main surfacesandwiched between the trenches. Therefore, as already described withreference to Embodiment 1, on-state voltage Vf can be reduced.

Further, in accordance with the present embodiment, it is not necessaryto apply a gate voltage to maintain off-state of the device. Namely, thedevice has a normally off type structure. Therefore, as compared with astructure which requires continuous application of gate voltage, thegate control circuit can be simplified in the present embodiment.

[Embodiment 6]

Referring to FIGS. 31 to 33, the semiconductor device of the presentembodiment differs from Embodiment 5 in that p⁺ isolation impurityregion 23 is formed. The p⁺ isolation impurity region 23 is formed tosurround planer region of diode forming region and to be in contact withtrenches 9. The p⁺ isolation impurity region 23 is made deeper than thetrench 9.

Except these points, the present embodiment is the same as Embodiment 5.Therefore, corresponding portions are denoted by the same referencecharacters, and description thereof is not repeated.

The method of manufacturing p⁺ isolation impurity region 23 isapproximately similar to the method described with reference to FIGS. 14to 16. Therefore, description thereof is not repeated.

Referring to FIG. 33, the p⁺ isolation impurity region 23 is connectedto cathode electrode 17 by an inverted layer formed around gateelectrode layer 13, when a negative voltage is applied to gate electrodelayer 13. Therefore, the pn junction formed by p⁺ isolation impurityregion 23 and n⁻ region 5 is reversely biased. Therefore, main breakdownvoltage maintaining capability of the device can be enhanced.

According to a semiconductor device of the present embodiment, p typeimpurity region 23 is formed deeper than trench 9 to surround diodeforming region, as shown in FIGS. 32 and 33. Therefore, p⁺ isolatedregion 23 is electrically isolated from the diode at on-state, on-statevoltage can be maintained low. And, p⁺ isolated region 23 iselectrically connected cathode electrode 17 at off-state, breakdownvoltage can be improved.

[Embodiment 7]

Referring to FIGS. 34 to 36, the semiconductor device in accordance withthe present embodiment differs from Embodiment 5 in that a p⁺ contactregion 31 is provided. The p⁺ contact region 31 is formed to be adjacentto cathode region 7 with trench 9 c or 9 d interposed, and iselectrically connected to cathode electrode 17. The p⁺ contact region 31has an surface impurity concentration of at least 1×10¹⁷ cm⁻³. The p⁺contact region 31 and n⁺ cathode region 7 are arranged alternately, withthe trenches interposed. Further, the number of trenches 9 a, 9 b, . . .extending parallel to each other can be arbitrarily selected.

Other structures are the same as those of Embodiment 5. Therefore,corresponding portions are denoted by the same reference characters anddescription thereof is not repeated.

The method of manufacturing the semiconductor device in accordance withthe present embodiment will be described.

The method of manufacturing in accordance with the present embodimentfirst includes the same steps as Embodiment 1 shown in FIGS. 4 to 8.Thereafter, referring to FIG. 37, p⁺ contact region 31 is formed at thesurface of n⁻ region 5 sandwiched between trenches 9 b and 9 c extendingparallel to each other, by photolithography process, ion implantationand diffusion, for example.

Referring to FIG. 38, through the same steps as shown in FIGS. 30 and31, p type base region 41 and n⁺ cathode region 7 are formed adjacent top⁺ contact region 31 with trenches 9 b and 9 c interposed. Subsequentsteps are the same as those of Embodiment 1. Therefore, descriptionthereof is not repeated.

In the present embodiment, since p⁺ contact region 31 is formed to beadjacent to n⁺ cathode region 7 with trench 9 interposed, turn-off timecan be reduced, as described with reference to Embodiment 3.

[Embodiment 8]

Referring to FIGS. 39 to 41, the semiconductor device in accordance withthe present embodiment differs from Embodiment 7 in that p⁺ isolationimpurity region 23 is formed. The p⁺ isolation impurity region 23 isprovided to surround, two dimensionally, the diode forming region, to bein contact with trench 9. The p⁺ isolation impurity region 23 is formedto be deeper than the trench 9.

Other structures are the same as those of Embodiment 7. Therefore,corresponding portions are denoted by the same reference characters anddescription thereof is not repeated.

The method of manufacturing p⁺ isolation impurity region 23 of thesemiconductor device in accordance with the present embodiment is thesame as the method shown in FIGS. 14 to 16 described above.

Referring to FIG. 41, the p⁺ isolation impurity region 23 is connectedto cathode electrode 17 by an inverted layer formed around gateelectrode layer 13, when a negative voltage is applied to gate electrodelayer 13. Therefore, the pn junction formed by p⁺ isolation impurityregion 23 and n⁻ region 5 is reversely biased. Therefore, main breakdownvoltage maintaining capability of the device can be enhanced.

According to a semiconductor device of the present embodiment, p typeimpurity region 23 is formed deeper than trench 9 to surround diodeforming region, as shown in FIGS. 40 and 41. Therefore, p⁺ isolatedregion 23 is electrically isolated from the diode at on-state, on-statevoltage can be maintained low. And, p⁺ isolated region 23 iselectrically connected cathode electrode 17 at off-state, breakdownvoltage can be improved.

[Embodiment 9]

Referring to FIGS. 42 to 44, the present embodiment shows an examplewhich includes a diode structure. The diode has a stacked structure ofp⁺ collector region 1, an n type buffer region 3, an n⁻ region 5 and ann⁺ cathode region 7. Trench 9 is formed from the surface of n⁺ cathoderegion 7 through n⁺ cathode region 7 to reach n⁻ region 5. At thesubstrate surface, p⁺ contact region 62 is provided to be in contactwith trench 9. Immediately below p⁺ contact region 62, there is providedn⁻ region 61 to be in contact with trench 9 and p⁺ contact region 62.

The p⁺ contact region 62 has surface impurity concentration of at least1×10¹⁷ cm⁻³, and n⁻ region 61 has impurity concentration of, forexample, 1−10¹² cm⁻³ to 1×10¹⁷ cm⁻³, which is lower than that of n⁺cathode region 7.

Other structures are the same as those of Embodiment 1. Therefore,corresponding portions are denoted by the same reference characters anddescription thereof is not repeated. The method of manufacturing asemiconductor device in accordance with the present embodiment will bedescribed.

Referring to FIG. 45, first, p⁺ collector region 1, n type buffer region3 and n⁻ region are formed stacked in this order. At the surface of n⁻region 5, an epitaxially grown layer having low concentrationcorresponding to n⁻ region 61 is formed, and thereafter selective ionimplantation, diffusion and the like are performed, so that anisland-shaped n⁻ region 61 is left.

Referring to FIG. 46, at a region between n⁻ regions 61, n⁺ cathoderegion 7 is formed by ion implantation and diffusion, for example.

The depth of diffusion of cathode region 7 is made approximately thesame as the depth of diffusion of n⁻ region 61.

Referring to FIG. 47, at a substrate surface above n⁻ region 61, p⁺contact region 62 is formed by ion implantation and diffusion, forexample. The p⁺ contact region 62 is formed shallower than n⁺ cathoderegion 7.

Referring to FIG. 48, a trench 9 a is from the substrate surface toreach n⁻ region 5 through p⁺ contact region 62 and n⁻ region 61.Thereafter, several steps as in Embodiment 1 are carried out, and thusthe semiconductor device shown in FIG. 44 is completed.

Here, n⁻ region 61 should preferably be formed to have impurityconcentration lower than n⁻ region 5. However, if n⁻ region 5 hassufficiently low impurity concentration, n⁻ region 61 may be formed byleaving n⁻ region 5.

The method of control of the semiconductor device in accordance with thepresent embodiment will be described. First, on-state is realized byapplying a positive voltage to gate electrode layer 13. AT this time, ann type accumulation region 65 having high electron density is formed astrench 9, as shown in FIG. 49. Therefore, present embodiment works insame manner of Embodiment 1.

Off-state can be realized by applying a negative voltage to gateelectrode layer 13. When a negative voltage is applied to gate electrodelayer 13, similar to Embodiments 1 to 8 described above, n⁺ accumulationlayer (channel) which is an electron current path is eliminated, thusthe current path becomes pinched off, and the device becomes turned off.Further, n⁻ regions 5 and 61 in contact with trench 9 are turned to p⁺inversion regions.

In order to reduce turn-off time, it is necessary to quickly extractminority carriers (here, hole). In the present embodiment, the holes,which are the minority carriers are extracted through the path of p⁺inversion region and p⁺ contact region 62 generated around trench 9. Asalready described with reference to Embodiment 2, the turn-off time canbe reduced in the present embodiment, also.

Referring to FIG. 49, at on-state, there is generated an n typeaccumulation channel region 65 having high electron concentration aroundtrench 9, and n type accumulation region 65 is regarded as an extensionof n⁺ cathode region 7. Namely, it is considered that effective cathoderegion becomes large. Thus cathode area, which is the area of contactbetween n⁺ cathode region 7 and n⁻ region 5 is increased. This enhancesinjection efficiency of electron, and this can reduce on-state voltageVf.

[Embodiment 10]

Referring to FIGS. 50 to 52, the structure of the semiconductor devicein accordance with the present embodiment differs from Embodiment 9 inthat p⁺ isolation impurity region 23 is provided. The p⁺ isolationdiffusion region 23 is formed to surround, two dimensionally, the diodeforming region, and to be in contact with trench 9. The p⁺ isolationimpurity region 23 is made deeper than trench 9.

The method of manufacturing a semiconductor device in accordance withthe present embodiment will be described.

The method of manufacturing a semiconductor device in accordance withthe present embodiment first includes the same steps as Embodiment 2shown in FIGS. 14 and 15. Thereafter, the step shown in FIG. 45 isperformed, and the state of FIG. 53 is attained. Thereafter, similarsteps as in Embodiment 1 are performed, and the semiconductor deviceshown in FIG. 52 is completed.

Referring to FIG. 52, the p⁺ isolation impurity region 23 is connectedto cathode electrode 17 by an inverted layer formed around gateelectrode layer 13, when a negative voltage is applied to gate electrodelayer 13. Therefore, the pn junction formed by p⁺ isolation impurityregion 23 and n⁻ region 5 is reversely biased. Therefore, main breakdownvoltage maintaining capability of the device can be enhanced.

According to a semiconductor device of the present embodiment, p typeimpurity region 23 is formed deeper than trench 9 to surround diodeforming region, as shown in FIGS. 51 and 52. Therefore, p⁺ isolatedregion 23 is electrically isolated from the diode at on-state, on-statevoltage can be maintained low. And, p⁺ isolated region 23 iselectrically connected cathode electrode 17 at off-state, breakdownvoltage can be improved.

Here, the trench 9 formed in respective embodiments may be arrangedconcentrically as shown in FIGS. 54 to 56, for example.

The planer structure shown in FIG. 54 corresponds to Embodiments 2 and6. The cross section taken along the line L-L′ of FIG. 54 corresponds toschematic cross sectional views of FIGS. 13 and 33.

The planer structure shown in FIG. 55 corresponds to Embodiments 4 and8. The cross section taken along the line M-M′ of FIG. 55 corresponds toschematic cross sectional views of FIGS. 25 and 41. The number oftrenches 9 shown in FIGS. 25 and 41 may be arbitrarily selected.

The planer structure shown in FIG. 56 corresponds to Embodiment 10. Thecross section taken along the line N-N′ of FIG. 56 corresponds to theschematic cross sectional view of FIG. 56.

[Embodiment 11]

Referring to FIG. 57, the semiconductor device in accordance with thepresent embodiment relates to an IGBT example. The structure of thesemiconductor device in accordance with the present embodiment isdifferent especially in the shape of the trenches, from the structure ofthe semiconductor device shown in FIG. 98. More specifically, the trench113 in the present embodiment is made deeper than the trench 413 shownin FIG. 98. The depth T₁₁ of trench 113 is from 5 to 15 μm and the widthW₁₁ is 0.8 to 3.0 μm. The pitch P₁₁ between the trenches 113 is, forexample, 4 μm.

As for the semiconductor device of the first conductivity type, in adevice having a breakdown voltage in the order of several hundred V, anepitaxially grown substrate having low impurity concentration of n typeof several ten Ω is used as n⁻ substrate (n⁻ region) 105. In the devicehaving the breakdown voltage in the order of several thousand V, an n⁻substrate 105 having high specific resistance of at least 100 Ωcm andlow impurity concentration of n type is used. More specifically, asilicon polycrystalline substrate manufactured by FZ (Floating Zone)method to have the thickness of about 600 μm of about 350 Ωcm, which isirradiated with neutral line and has its resistivity adjusted by heattreatment, is used.

Further, in order to control resistivity, n or p type impurity is dopedin the substrate having high resistance. However, in the on state of abipolar device, electrons and holes which are carriers, are sufficientlyaccumulated in the high resistance layer, causing conductivitymodulation. Therefore, the substrate may be sometimes regarded as anintrinsic semiconductor.

In the present embodiment, the thickness T₁₀₁ of p⁺ collector region is,for example, 3 to 350 μm, the thickness T₁₀₃ of n⁺ buffer region 103 is,for example, 8 to 30 μm, the thickness T₁₀₅ of n⁻ region 105, forexample, 40 to 600 μm, the thickness T₁₀₇ of p type base region 107 is,for example, 2.0 to 3.5 μm, and the thickness T₁₀₉ of n⁺ emitter region109 is, for example, 0.5 to 1.5 μm.

The p type base region 107 has only to be formed to be shallower thantrench 113, and more specifically, the depth thereof is about 3 μm.

As for the impurity concentrations of various portions, it is 1×10¹⁶cm⁻³ to 5×10²¹ cm⁻³ in p⁺ collector region, 1×10¹³ cm⁻³ to 1×10¹⁹ cm⁻³in n⁺ buffer region 103, 1×10¹² cm⁻³ to 1×10¹⁴ cm⁻³ in n⁻ region 105,the peak concentration of p type base region 107 is 1×10¹⁵ cm⁻³ to1×10¹⁷ cm⁻³, the concentration of p⁺ contact region 111 at the substratesurface is at least 1×10¹⁸ cm⁻³, and the concentration of n⁺ emitterregion 109 at the substrate surface is 1×10¹⁹ cm⁻³ to 1×10²⁰ cm⁻³.

Other structures are approximately the same as those of the third priorart example shown in FIG. 98. Therefore, corresponding portions aredenoted by the same reference characters and description thereof is notrepeated.

The method of manufacturing the semiconductor device in accordance withthe present embodiment will be described.

First, referring to FIG. 58, p⁺ collector region 101, n⁺ buffer region103 and n⁻ region 105 are formed stacked successively. Thereafter, atthe surface of n⁻ region 105, p type base region 107 and n⁺ emitterregion 109 are formed.

Referring to FIG. 59, by anisotropic etching of the substrate, trench113 having its bottom reaching n⁻ region 105 through n⁺ emitter region109 and p type base region 107 is formed. The trench 113 is formed tohave the width of 0.8 to 3.0 μm and the depth of 5.0 to 15.0 μm, bycontrolling etching. More preferably, the trench has a depth of at least10.0 μm.

Referring to FIG. 60, by, for example, thermal oxidation, a gate oxidefilm 115 of silicon oxide film is formed along the inner wall surface ofand covering the surface of trench 113.

Before the formation of gate oxide film 115 and after the formation oftrench 113, isotropic plasma etching (i.e. chemical dry etching) may beperformed followed by sacrificial oxidation to form a silicon oxide filmonce on the inner wall surface or the like of trench 113, so as toimprove MOS characteristic and characteristic of gate oxide film.

Referring to FIG. 61, a doped polysilicon layer, doped with an n typeimpurity such as phosphorous is formed to fill trench 113. Byanisotropically etching the doped polysilicon layer, gate electrodelayer 117 is formed which fill the trench 113 and has its upper endprojecting from trench 113.

Referring to FIG. 62, at a region between trenches 113, a p⁺ contactregion 11 for reducing contact resistance is formed by implantation of ptype ion and diffusion, for example. The p⁺ contact region 111 must haveconcentration of at least 1×10²⁰ cm⁻³, and it may have approximately thesame depth as n⁺ emitter region 109. An interlayer insulating layer 119formed of an CVD oxide film such as BPSG is formed to cover the upperend of gate electrode layer 110 projecting from trench 113.

Thereafter, cathode electrode 121 is formed to be electrically connectedto n⁺ emitter region 109, and p⁺ contact region 111, anode electrode 123is formed to be electrically connected to p⁺ collector region 101, andthus the semiconductor device shown in FIG. 57 is completed.

The method of controlling on and off states by gate electrode layer 117in the semiconductor device in accordance with the present embodiment isapproximately the same as in the third prior art example shown in FIG.98. Therefore, description thereof is not repeated.

In view of the result shown in FIG. 22, the inventors have found thatthe larger the ratio Rn, the smaller the on-state voltage Vf.Especially, it was found that when the ratio Rn is at least 0.4,on-state voltage Vf becomes low and stable. Further, it was found thatratio Rn of 0.7 or higher is more preferable. When the ratio Rn in theIGBT structure of the prior art example (FIG. 98) is evaluated, it wasfound that the ratio Rn was smaller than 0.4, which means that electronssupplying capability from cathode surface is very poor.

In the semiconductor device in accordance with the present embodiment,the depth of trench 113 is at least 5 μm which is deeper than the thirdprior art example shown in FIG. 98, and hence n⁺ accumulation region 425a generated in on-state shown in FIG. 99 has larger distribution ascompared with the third prior art example. Therefore, the effectivecathode region constituted by n⁺ accumulation region 425 a and n⁺emitter region 109 becomes larger than the third prior art example, andhence larger effective cathode area can be ensured. Since effectivecathode area shown in FIG. 22 is enlarged, the ratio Rn (=n/(n+p)) isincreased. More specifically, the ratio Rn shown in FIG. 22 can be setto be 0.4 or higher, which value can not be obtained in the third priorart example shown in FIG. 98. Since the ratio Rn can be made higher thanthe third prior art example, on-state voltage Vf can be decreased fromthe third prior art example.

Here, the area p which is an element defining the ratio Rn refers to thecontact area of p type base region 107 and n⁻ region 105, which isrepresented by a thick line in FIG. 57.

The depth T₁₁ should preferably be at least 10 μm so as to decreaseon-state voltage Vf.

According to the semiconductor device of the present embodiment, thecontrol method by the gate electrode layer 117 is of voltage controlledtype. Therefore, in the semiconductor device in accordance with thepresent embodiment, as compared with the first and second prior artexamples, the structure of the gate control circuit can be simplified,the whole system can be reduced in size, simplified and energyconsumption can be reduced.

[Embodiment 12]

Referring to FIG. 63, the structure of the semiconductor device inaccordance with the present embodiment differs from the structure of thesemiconductor device in accordance with Embodiment 11 in the structureof the region 31 between the trenches and the structure of gateelectrode layer.

In the region sandwiched between trenches 113 a and 113 b and in theregion sandwiched between trenches 113 c and 113 d, p type base region107, n⁺ emitter region 109 and p⁺ contact region 111 are formed as inEmbodiment 11. In the region sandwiched between trenches 113 b and 113c, p type base region 107 and the like are not formed, and only n⁻region 105 is positioned.

Gate electrode layer 117 filling trench 113 b and gate electrode layer117 filling trench 113 c are formed integrally by a conducting portion117 a, and they are electrically connected to each other. The conductingportion 117 a is formed on the region sandwiched between trenches 113 band 113 c, with an insulating film 129 interposed.

Other structures are approximately the same as those of Embodiment 11.Therefore, corresponding portions are denoted by the same referencecharacters, and description thereof is not repeated.

The above described structure will be hereinafter referred to as MAE(MOS Accumulated Emitter) structure.

The structure of the present embodiment is in line symmetry with respectto both lines R-R′ and S-S′ of FIG. 63. Therefore, a unit cell may beregarded as a structure between R-R′ line and S-S′ line, or it may beconsidered as a structure between one R-R′ line and another R-R′ line.Here, for convenience of calculation of the ratio Rn, the formerstructure, that is, the structure between R-R′ line and S-S′ line isregarded as a unit cell.

The method of manufacturing a semiconductor device in accordance withthe present embodiment will be described.

Referring to FIG. 64, p⁺ collector region 101, n type buffer region 103and n⁻ region 105 are formed stacked successively. Thereafter, at thesurface of n⁻ region 105, p type base region 107 and n⁺ emitter region109 are selectively formed.

Referring to FIG. 65, by anisotropic dry etching used in commonsemiconductor process, trenches 113 a to 113 d are formed at thesubstrate surface which will be the first main surface. Each trench isformed to have the width of 0.8 to 3.0 μm and the depth of 5 to 15 μm byetching control, as in Embodiment 11. The trenches are formed such thatin the region sandwiched between trenches 113 a and 113 b and in theregion sandwiched between trenches 113 c and 113 d, p type base region107 and n⁺ emitter region 109 are positioned, and that in the regionsandwiched between trenches 113 b and 113 c, only the n⁻ region 105 ispositioned. In this state, the n⁺ emitter region 109 is positioned alongthe sidewall of the trench.

Gate oxide film 115 of silicon oxide film is formed along the inner wallsurface of and to cover the surface of trenches 113 a to 113 d, by, forexample, thermal oxidation.

Before the formation of gate oxide film 115 and after the formation ofeach trench, isotropic plasma etching may be performed, followed bysacrificial oxidation to form a silicon oxide film on the inner wallsurface of each trench, so that MOS characteristic and thecharacteristic of the gate oxide film 115 can be improved.

Referring to FIG. 66, a doped polysilicon layer doped with an n typeimpurity such as phosphorus is formed to fill each trench. Byanisotropic etching of doped polysilicon layer, gate electrode layer 117is formed to fill each trench with its upper end projecting from eachtrench. Gate electrode layers 117 filled in trenches 113 b and 113 c areformed integrally by conducting portion 117 a to be electricallyconnected to each other. The conducting portion 117 a is formed at thesurface region sandwiched between trenches 113 b and 113 c with aninsulating film interposed.

Thereafter, at a region between trenches 113 a and 113 b and at aportion between trenches 113 c and 113 d, p⁺ contact regions 111 areformed to reduce contact resistance by selective impurity implantationof p type and diffusion, for example.

Referring to FIG. 67, an interlayer insulating layer 119 formed of a CVDoxide film such as BPSG is formed to cover the upper end of gateelectrode layer 117 protruding from each trench.

Thereafter, cathode electrode 121 is formed to be electrically connectedto n⁺ emitter region 109 and p⁺ contact region 111, and anode electrode123 is formed to be electrically connected to p⁺ collector region 101,and thus the semiconductor device shown in FIG. 63 is completed.

The method of controlling on and off states by gate electrode layer 117in the present embodiment is approximately the same as the third priorart example shown in FIG. 98. Therefore, description thereof is notrepeated.

However, when a positive voltage is applied to gate electrode layer 117in on-state, there will be n⁺ accumulation layer 125 b generated asshown in FIG. 68.

In the semiconductor device in accordance with the present embodiment,the conducting portion 117 a is electrically connected to gate electrodelayers 117 filling trenches 113 b and 113 c, as shown in FIG. 63.Therefore, when a positive voltage is applied to gate electrode layer117 in on-state, the positive voltage is also applied to conductingportion 117 a. The conducting portion 117 a opposes to n⁻ region 105sandwiched between trenches 113 b and 113 c, with insulating film 129interposed. Therefore, when the positive voltage is applied toconductive layer 117 a, there will be n⁺ accumulation region 125 bgenerated at the surface region sandwiched between trenches 113 b and113 c, as shown in FIG. 68. Since it is possible to generate n⁺ surfaceregion 125 b even at the surface region between trenches 113 b and 113c, the effective cathode area in the unit cell can be enlarged from thatof Embodiment 11. Therefore, efficiency in injecting electrons on thecathode side can further be improved and on-state voltage Vf can furtherbe reduced. Further, the ratio Rn becomes higher than 0.4 and close to1.

In the semiconductor device in accordance with the present embodiment,the control method by gate electrode layer 117 is of voltage controlledtype. Therefore, in the semiconductor device in accordance with thepresent embodiment, as compared with the first and second prior artexamples, the structure of the gate control circuit can be simplified,as already described. Accordingly, the whole system can be reduced insize and simplified and energy consumption can be reduced.

Further, since the depth of trench 113 is at least 5 μm, on-statevoltage Vf can be made lower than the third prior art example, asalready described with reference to Embodiment 11.

[Embodiment 13]

Referring to FIG. 69, the semiconductor device in accordance with thepresent embodiment differs from Embodiment 12 in that it has a second ptype base region 131. The second p type base region 131 is formed, forexample, at a surface region between trenches 113 b and 113 c. Thesecond p type base region 131 is formed, for example, at every otherregion between the trenches.

Further, the second p type base region 131 has lower impurityconcentration than p type base region 107.

Other structures are approximately the same as those of Embodiment 12.Therefore, corresponding portions are denoted by the same referencecharacters and description thereof is not repeated.

The method of manufacturing a semiconductor device in accordance withthe present embodiment will be described.

Referring to FIG. 70, p⁺ collector region 101, n⁺ buffer region 103 andn⁻ region 105 are formed stacked successively. At the surface of n⁻region 105, p type base region 107, the second p type base region 131and. n⁺ emitter region 109 are each formed by ion implantation anddiffusion. Here, the second p type base region 131 is formed to havelower impurity concentration than p type base region 107.

Thereafter, by photolithography and etching (RIE), trenches 113 a to 113d having its bottom region reaching n⁻ region 105 through p type baseregion 107, n⁺ emitter region 109 and second p type base region 131 areformed. Each trench is formed to have the width of 0.8 to 3.0 μm and thedepth of 5 to 15 μm.

Thereafter, by thermal oxidation, for example, gate oxide film 115 ofsilicon oxide film is formed along the inner wall surface of eachtrench. Before formation of gate oxide film 115 and after the formationof each trench, isotropic plasma etching may be performed followed bythe sacrificial oxidation to form silicon oxide film on the inner wallsurface of each trench, so that MOS characteristics and characteristicsof gate oxide film 115 can be improved.

Thereafter, similar steps as in Embodiment 12 shown in FIGS. 66 and 67above are performed, and the semiconductor device shown in FIG. 69 iscompleted.

The method of controlling on and off states by gate electrode 117 of thepresent embodiment is approximately the same as described with referenceto the third prior art example. Therefore, description thereof is notrepeated.

However, when a positive voltage is applied to gate electrode layer 117in on-state, there will be n⁺ accumulation region 125 c having highelectron density as shown in FIG. 71. Further, thyristor operationoccurs in the region between trenches 113 b and 113 c.

In the semiconductor device in accordance with the present embodiment,as in Embodiment 12, n⁺ accumulation region 125 c may be generated alsoat the surface region between trenches 113 b and 113 c, as shown in FIG.71. Therefore, as in Embodiment 12, efficiency in injecting electrons ofthe cathode side can be enhanced, and on-state voltage Vf of the diodecan be reduced. Consequently, ratio Rn becomes 0.4 or higher, closer to1.

Further, since the second p type base region 131 has lower concentrationthan p type base region 107, thyristor operation occurs at the regionsandwiched between trenches 113 b and 113 c. As a result, ON voltage canbe decreased when rated current is conducted.

Further, at off-state, a negative voltage is applied to gate electrodelayer 117. Therefore, at the portion along the sidewall of trenches 113b and 113 c of the second p type base region 131 and at the surfaceregion of the substrate, p⁺ inversion region s are formed. Therefore, asalready described, holes which are carriers tend to be more easilyextracted from p⁺ inversion region, making shorter than turn-off timeand smaller the tail current. Since tail current at the time of turn offis reduced, turn off loss E_(off) can also be reduced.

In the semiconductor device in accordance with the present embodiment,the control method by gate electrode layer 117 is of voltage controlledtype. Therefore, in the semiconductor device in accordance with apresent embodiment, the structure of the gate control circuit can besimplified as compared with the first and second prior art examples.Further, the whole system can be reduced in size, simplified and powerconsumption can be reduced.

In the semiconductor device in accordance with the present embodiment,as in Embodiment 11, the thickness T₁₃ of trenches 113 a-113 d is atleast 5 μm. Therefore, as already described with reference to Embodiment11, on-state voltage Vf can be lower than the third prior art example.

[Embodiment 14]

Referring to FIG. 72, the structure of the semiconductor device inaccordance with the present embodiment differs from the structure ofEmbodiment 11 in that p⁻ base region 133 is provided. The p⁻ base region133 is positioned below p type base region 107 and arranged along thesidewall of trench 113. The p⁻ base region 133 has impurityconcentration of 1×10¹⁴ cm⁻³ to 1×10¹⁶ cm⁻³.

Other structures are approximately the same as those of Embodiment 11.Therefore, corresponding portions are denoted by the same referencecharacters and description thereof is not repeated.

In the semiconductor device in accordance with the present embodiment,when a negative voltage is applied to gate electrode layer 117 atoff-state, a p⁺ inversion layer is formed at a portion along trench 113in p⁻ base region 133. Therefore, at the time of turn off of the device,extraction of the holes, which are carriers can be carried out smooth,resulting in improved switching characteristic.

When a positive voltage is applied to gate electrode layer 117 aton-state, an inversion n layer is formed at a portion along trench 113in p⁻ base region 133. Therefore, the ratio Rn can be kept high.

Accordingly, while ratio Rn is kept high, the switching characteristiccan be improved.

Further, in the semiconductor device in accordance with the presentembodiment, the control method by gate electrode layer 117 is of voltagecontrolled type. Therefore, in the semiconductor device in accordancewith the present embodiment, the structure of the gate control circuitcan be simplified as compared with the first and second prior artexamples, as already described above. Further, the whole system can bereduced in size and simplified, and energy consumption can be reduced.

Further, in a semiconductor device in accordance with the presentembodiment, the depth of trench 113 is at least 5 μm, as in Embodiment11.

Therefore, as in Embodiment 11, on-state voltage Vf can be made lowerthan the third prior art example.

[Embodiment 15]

FIG. 73 is a cross sectional view schematically showing a portion of thestructure shown in FIG. 57.

Referring to FIG. 73, the inventors have found that the ratio Rn can beapproximated using dimensions of respective portions of IGBT. The ratioRn can be represented as Rn=n/(n+p), as already described with referenceto Embodiment 3. Here, the term n represents the area of the poriondenoted by the thick line in FIG. 73. More specifically, the area n isthe sum of the area of contact of n⁺ accumulation region 125 a, n⁻region 105 and p type base region 107 and the area of the contact of noemitter region 109 with p type base region 107, at on-state. Meanwhile,p represents the area of contact between p type base region 107 and n⁻region 105, as already described.

Here, the width of n⁺ accumulation region 125 a is very small.Therefore, when the width of trench 113 is represented by Wt, depth oftrench 113 from cathode surface (first main surface) is represented byDt, the depth of n⁺ emitter region from cathode surface is representedby De, the width of n⁺ emitter region 109 in the direction from onetrench 113 to another trench 113 is represented by We, the width of ptype base region 107 from one trench 113 to another trench 113 isrepresented by Wp and the depth of p type region 107 from the cathodesurface is represented by Dp, n and p can be given by the followingequations.n=2(We+Dt−De)+Wtp=Wp

By substituting the above equations for the ratio Rn, the ratio Rn canbe given by the following equations.${Rn} = \frac{{2\quad\left( {{We} + {Dt} - {De}} \right)} + {Wt}}{{2\left( {{We} + {Dt} - {De}} \right)} + {Wt} + {Wp}}$

If we represent the pitch of trench 113 as Pt (FIG. 74), it holds thatWt+Wp=Ptand therefore, ratio Rn can be transformed to${Rn} = {\frac{{2\quad\left( {{We} + {Dt} - {De}} \right)} + {Wt}}{{2\left( {{We} + {Dt} - {De}} \right)} + {Pt}}.}$

Here, when areas n and p are to be calculated, it is correct to usenumerical values obtained by multiplying total length (=length L oftrench×number of trenches) in the depth direction of FIG. 73. However,in a structure in which strip shaped trenches extend parallel to eachother, the total length in the depth direction is equally multiplied byrespective terms. Therefore, the value can be approximated by the abovedescribed expression, with the total length omitted.

Further, referring to FIG. 73, the bottom surface of trench 113 isassumed to be planer, for convenience of description. However, in actualdevices, the bottom of trench 113 is generally rounded, to improve gatebreakdown voltage, as shown in FIG. 57. Therefore, in calculating theratio Rn, a coefficient larger than 1 is multiplied by the area Wt ofthe trench bottom. However, it is omitted for simplicity of description.

More specifically, if a deep trench gate is to be formed, when Pt=5.5μm, Dt=15 μm, Wt=1 μm, De=1 μm, We=0.8 μm, the ratio Rn would be$\begin{matrix}{{Rn} = {\left\lbrack {1 + {\left( {0.8 + 15 - 1} \right) \times 2}} \right\rbrack/\left\lbrack {5.5 + {\left( {0.8 + 15 - 1} \right) \times 2}} \right\rbrack}} \\{= {{15.8/20.3} = 0.78}}\end{matrix}$and hence large ratio Rn is obtained.

[Embodiment 16]

Referring to FIG. 74, by the equation of Rn above, it can be understoodthat it is effective to increase the width Wt of trench 113 to increasethe ratio Rn, even when the trench 113 is shallow, that is, the depth Dtof trench 113 is small.

More specifically, if Pt=9 μm, Dt=5 μm, Wt=6 μm, De=1 μm and We=0.8 μm,then $\begin{matrix}{{Rn} = {\left\lbrack {6 + {\left( {0.8 + 5 + 1} \right) \times 2}} \right\rbrack/\left\lbrack {9 + {\left( {0.8 + 5 + 1} \right) \times 2}} \right\rbrack}} \\{= {{19.6/22.6} = 0.87}}\end{matrix}$and hence, large ratio Rn is obtained.

[Embodiment 17]

The structure of the semiconductor device in accordance with the presentembodiment is approximately similar to the structure of Embodiment 12shown in FIG. 63. The structure is relatively complicated as comparedwith Embodiment 15 above, variables to be optimized are increased andthe steps of manufacturing becomes complicated. However, it isadvantageous in that larger ratio Rn is obtained and it is effective toreduce on voltage.

The method of controlling on and off states by the gate electrode layer117 in the present embodiment is approximately the same as in Embodiment12 above. Therefore, description thereof is not repeated.

Especially a positive voltage is applied to gate electrode layer 117 aton-state, n⁺ accumulation region 125 b is generated, as shown in FIG.68.

Here, if the structure between lines R-R′ and S-S′ is regarded as a unitcell, the area n will ben=2Dt−De+We+Wn+Wt.

As is apparent from this equation, in the semiconductor device inaccordance with the present embodiment, n⁺ accumulation region 125 b isalso generated at the surface region between trenches 113 b and 113 c,as shown in FIG. 68. Therefore, the effective cathode area in the unitcell can be enlarged than Embodiment 15. Therefore, injection efficiencyof electron on the cathode side can further be enhanced, and on-statevoltage Vf can further be reduced. Therefore, the ratio Rn can be made0.4 or higher, closer to 1.

The method of manufacturing the semiconductor device in accordance withthe present embodiment will be described. The manufacturing method inaccordance with the present embodiment will be described referring to anexample in which the device having the breakdown voltage in the order of4500 V is manufactured.

First, referring to FIG. 75, an n⁻ silicon substrate 105 having highresistivity of about 200 to 400 Ωcm is formed by the FZ method. On theanode side, which will be the second main surface of n⁻ siliconsubstrate 105, an n⁺ buffer region 103 having the thickness of about 10to 30 μm and having high impurity concentration of the firstconductivity type, that is, n type, and a p⁺ collector region (p⁺ anoderegion) 101 having the thickness of about 3 to about 10 μm and havinghigh impurity concentration of a second conductivity type, that is, ptype, are formed.

According to one method of manufacturing n⁺ buffer region 103, after ionimplantation of phosphorous having large coefficient of diffusion,drive-in is performed for 20 to 30 hours at a high temperature of 1200to 1250° C., so that peak concentration of n⁺ buffer region 103 afterthe final step is within the range of about 1×10¹⁶ to about 5×10¹⁷ cm⁻³and the depth is from about 10 μm to about 30 μm. Alternatively, vaporphase deposition by a gas obtained by bubbling PH₃ gas or POCl₃ may beused instead of ion implantation of phosphorus.

In another method of manufacturing n⁺ buffer region 103, a siliconcrystal layer is formed having approximately the same n type impurityconcentration as obtained by ion implantation by epitaxial growth.

The method of manufacturing p⁺ collector region 101 includes the methodperforming drive-in after ion implantation or vapor phase depositionwhich is similar to the method of manufacturing n⁺ buffer region 103,and a method of forming p type silicon crystal layer by epitaxialgrowth. However, in this case, boron or gallium is used as p typeimpurity. Therefore, the source gas for a vapor phase deposition issublimated gas of, for example, boron glass (B₂O₃ or the like) generatedby oxidation of BN (Boron Nitride) which is a solid source or B₂H₆ gas.The p⁺ collector region 101 is formed such that it has the depth of 3 to10 μm and it has peak concentration higher than the peak concentrationof n⁺ buffer region 103 after the final step.

Referring to FIG. 76, in a region sandwiched by trenches (denoted bydotted lines in the figure) which will be formed in subsequent steps,boron ions are selectively implanted, using a resist pattern 151 as amask. Consequently, p type base region 107 a of the second conductivitytype is formed at the first main surface of n⁻ silicon substrate 105.When the trenches are to be formed in stripes with small repetitioninterval (pitch) of about 3 to about 5 μm, it is necessary to preventinvasion of p type base region 107 a to such regions that do notconstitute the IGBT structure, by performing long heat treatment (forexample, 30 minutes to 7 hours at a relatively high temperature of 1100°C. to 1150° C.) for diffusing p type base region 107 a. Therefore, it isnecessary to introduce boron ions with p base implantation width Wp(imp) which is smaller than the repetition interval (Tr-pitch) of thetrenches.

Referring to FIG. 77, a resist pattern 152 is formed on the first mainsurface by common photolithography. By using resist pattern 152 as amask, n type impurity such as phosphorous, arsenic or antimony isintroduced by ion implantation, and thus n⁺ emitter region 109 a of thefirst conductivity type is formed. Thereafter, resist pattern 152 isremoved.

Referring to FIG. 78, by common photolithography, a resist pattern 153is formed on the first main surface. By using resist pattern 153,trenches 113 a to 113 b are formed as stripes with prescribed pitch byRIE method or other silicon anisotropic etching. Thereafter, fordiffusing p type base region 107 described above, relatively long heattreatment is performed for about 30 minutes to 7 hours at a relativelyhigh temperature of 1100° C. to 1150° C. By this heat treatment, p typebase region 107 a and n⁺ emitter region 109 are diffused. Thereafter,resist pattern 153 is removed.

The conditions for the above described heat treatment, such astemperature and time are determined such that the p type base region 107can be formed deep enough to meet the main breakdown voltage required ofthe manufactured device. More specifically, in the device having abreakdown voltage in the order of 4500 V, a p type base region 107 of atleast 2 μm is necessary below n⁺ emitter region 109. Therefore, thedepth of diffusion of p type base region 107 from the substrate surfaceis, in this case, the depth of diffusion of n⁺ emitter region 109 plusabout 2 μm. This is the reason why such heat treatment for a long periodof time at a high temperature is necessary.

In order to avoid such heat treatment at a high temperature for a longperiod of time, there is a method to implant ions deeper selectively, byusing high energy ion implantation in the step of ion implantation shownin FIG. 76. In that case, resist pattern 151 used as the mask is adaptedto have higher viscosity of about 300 to about 500 cp, higher than thenormal viscosity (of several ten cp (centipoise; unit of viscosity)).The resist pattern 151 is formed to have a thickness of several μm, sothat it can shield ions implanted with high energy of about 3 to about 5MeV. The range of boron ions in silicon when ions are implanted withhigh energy of this level is about 2 to about 4 μm. Therefore, thedesired depth of diffusion of p type base region 107 a can be obtained,hardly performing heat treatment.

If heat treatment for diffusing p type base region 107 is excessive orif the hole pattern of resist for selective implantation (diffusion) istoo large, p type base region 107 protrudes to such regions that do notinherently constitute IGBT structure, as shown in FIGS. 85 and 86. Insuch a case, the object of the present invention, that is, to improvethe device characteristic by enlarging the ratio Rn cannot be attained.

On the other hand, if heat treatment for diffusing p type base region107 is not sufficient or if the hole pattern of the resist for selectiveimplantation (diffusion) is too small, there will be a portion of n⁺emitter region 109 not covered by p type base region 107 at the IGBTstructure as shown in FIGS. 87 and 88, and in that case, main breakdownvoltage cannot be maintained.

Referring to FIG. 79, by sacrificial oxidation, oxide film 115 is formedon inner walls of trenches 113 a to 113 b. Thereafter, wet etching isperformed as shown in FIG. 80 and oxide film 115 is removed.

Referring to FIG. 81, by thermal oxidation, silicon oxide film 115 isformed on the inner walls of trenches 113 a to 113 d and on the firstmain surface. Silicon oxide film 115 is formed in accordance with gatebreakdown voltage, gate input capacitance and gate threshold voltagerequired of the device.

A conductive film 117 c of phosphorus doped polycrystalline silicon isformed on the first main surface to fill trenches 113 a to 113 d. Theconductive film 117 c has the thickness approximately similar to orlarger than the opening width of trenches 113 a to 113 d, and is formedby using a reduced pressure CVD apparatus or the like. Thereafter,conductive film 117 c is entirely etched (generally referred to as etchback) to have a relatively thin film thickness to facilitate processingin subsequent steps.

Thereafter, conductive film 117 c is selectively improved by commonphotolithography and dry etching, so as to leave a connecting portion ofthe surface interconnection for the control electrodes (gates).

Referring to FIG. 82, by this selective removal, control electrodelayers (gate electrode layers) 117 filling trenches 113 a to 113 d andhave portion 117 a extending on a region where IGBT structure is notformed with an insulating film 129 interposed are formed.

Referring to FIG. 83, by combining common photolithography and ionimplantation technique of p type impurity such as boron, p⁺ contactregion 111 of the second conductivity type is formed at the first mainsurface to be adjacent to n⁺ emitter region 109.

Referring to FIG. 84, a CVD silicon oxide film such as BPSG or a siliconnitride film are formed as interlayer insulating film 119 a to covergate electrode layer 117. A contact hole or a line-shaped contactportion is formed at interlayer insulating film 119 a. Thereafter, metalinterconnection such as aluminum is formed on the first main surface bysputtering, and thus the semiconductor device shown in FIG. 63 iscompleted.

The n⁺ emitter region 109 may not be formed by the process shown inFIGS. 77 and 78. Alternatively, it may be formed after the controlelectrode layer 117 shown in FIG. 82 is formed. When n⁺ emitter region109 is formed after the gate electrode layer 117 shown in FIG. 82 isformed, n⁺ emitter region 109 may be formed after the formation of p⁺contact region 111 shown in FIG. 83.

Alternatively, after the trenches 113 a to 113 d are formed in the stepof FIG. 78, isotropic plasma etching (chemical dry etching) may beperformed as disclosed, for example, in Japanese Patent Laying-Open Nos.6-012559 and 7-001347.

More specifically, trenches 113 a to 113 d are formed in the step ofFIG. 78, then isotropic plasma etching is performed as shown in FIG. 9,corners at the openings of trenches 113 a to 113 d are removed, andbottoms of the trenches are rounded. Thereafter, the deposition filmformed at the time of etching is removed by wet etching. Thereafter,oxide film 115 is formed on the inner walls of trenches 113 a to 113 dby sacrificial oxidation as shown in FIGS. 79 to 80, and oxide film 115is removed by wet etching.

Consequently, the shapes in and at the opening portion of the trenches113 a to 113 d are adjusted and at the same time, contaminated layer ordamaged layer caused by anisotropic etching can be removed.

At least one of sacrificial oxidation shown in FIG. 79 and isotropicplasma etching of low damage may be performed.

The semiconductor device in accordance with the present embodimentincludes complicated manufacturing steps as compared with Embodiment 15.However, it is not necessary to make trenches 113 a to 113 d extremelydeep or extremely wide. Therefore, the step of etching itself forforming the trenches and the step of filling trenches by dopedpolysilicon film using CVD method do not require a long time. Therefore,burden on the manufacturing apparatus is released. Therefore, generalcost efficiency is comparable with Embodiment 15.

[Embodiment 18]

Referring to FIG. 90, the structure of the present embodiment differsfrom the structure of Embodiments 12 and 17 shown in FIG. 63 in thestructure of gate electrode layer 117. More specifically, the gateelectrode layer 117 does not extend over the area where IGBT structureis not formed (hereinafter referred to as non-IGBT region). Morespecifically, on the non-IGBT region, cathode electrode 121 is formedonly with the insulating layer (insulating layer 129 and interlayerinsulating film 119) interposed.

Except this point, the structure is the same as those of Embodiments 12and 17. Therefore, corresponding portions are denoted by the samereference characters and description thereof is not repeated.

The method of manufacturing a semiconductor device in accordance withthe present embodiment will be described.

The method of manufacturing in accordance with the present embodimentfirst includes the same steps as Embodiment 17 shown in FIGS. 75 to 81.Thereafter, referring to FIG. 91, by common photolithography and dryetching, gate electrode layer is patterned so as not to extend over thenon-IGBT region and to protrude over the first main surface.

Thereafter, the same steps as in Embodiment 17 are performed, and thesemiconductor device shown in FIG. 90 is completed.

If gate electrode layer 117 is adapted not to extend over the non-IGBTregion, the simplicity of the manufacturing steps is comparable toEmbodiment 17 in which gate electrode layer extends over the non-IGBTregion.

As compared with Embodiment 17, in the semiconductor device inaccordance with the present embodiment, gate electrode layer is notextended over the non-IGBT region. In the on state, n⁺ emitter region(accumulation region) extended over the first main surface of thenon-IGBT region is not formed, and hence the ratio Rn in the on statebecomes smaller. However, by making smaller the pitch of trenchessandwiching the non-IGBT region as compared with the pitch of thetrenches sandwiching IGBT region, the ratio of the enlarged n⁺ emitterregion (accumulation region) in the ratio Rn becomes smaller. Therefore,approximately the same ratio Rn as in Embodiment 17 can be obtained.

Further, at a portion where the gate electrode layer extends over thefirst main surface, interlayer insulating film 119 has thinner filmthickness. This leads to defective breakdown voltage between gateelectrode layer 117 and emitter electrode 121, resulting in decreasedproduction yield. In view of the production yield, it is preferable thatthe gate electrode extends as small as possible over the first mainsurface. Therefore, the semiconductor device in accordance with thepresent embodiment is effective in industrial application as comparedwith the structure of Embodiment 17.

[Embodiment 19]

Referring to FIG. 92, in the present embodiment, as compared withEmbodiments 12 and 17 shown in FIG. 63, a plurality of non-IGBT regionsare arranged in a region between two IGBT forming regions.

Referring to FIG. 92, the structure of the present embodiment is in linesymmetry with respect to lines R-R′ and S-S′. Therefore, a structurebetween lines R-R′ and S-S′ may be regarded as a unit cell, or thestructure between one line R-R′ and another R-R′ may be considered asthe unit cell. Here, the latter structure, that is, the structurebetween one R-R′ line another R-R′ line is regarded as a unit cell.Therefore, in the unit cell, the number of non-IGBTd regions sandwichedbetween two IGBT forming regions is 3. In other words, between two IGBTforming regions, there are four trenches 115 sandwiching non-IGBTregions.

The larger the number of non-IGBT regions between two IGBT formingregions, the closer the ratio Rn to 1. However, though it depends tosome extent on the pitch between trenches and depth of the trench, ifthe number of non-IGBT regions between two IGBT forming regions is outon the range of 2 to 4, the ratio Rn begins to saturate. Further, the n⁺emitter region (n⁺ accumulation region) extended in the on state isformed only in the close vicinity at the interface between siliconsubstrate and gate oxide film (in the range of up to about 100 Å).Therefore, if the extended n⁺ emitter region (accumulation region)becomes too long, the resistance of the accumulation region will also beincreased to a innegligible level. Therefore, the number of non-IGBTregion between two IGBT forming regions should preferably be at most 4.In other words, the number of trenches 115 positioned between two IGBTforming regions should preferably be at most 5.

The semiconductor device in accordance with the present embodiment canbe manufactured through approximately the same steps as Embodiment 17.

[Embodiment 20]

Referring to FIG. 93, the present embodiment differs from Embodiment 19shown in FIG. 92 in the structure of gate electrode layer 117. In thepresent embodiment, gate electrode layer 117 cannot extend to thenon-IGBT region.

Other structures are approximately the same as those of Embodiment 19.Therefore, corresponding portions are denoted by the same referencecharacters and description thereof is not repeated.

The semiconductor device of the present embodiment can be manufacturedthrough approximately the same steps as Embodiment 18.

In the semiconductor device of the present embodiment, gate electrodelayer 117 does not extend over the non-IGBT region. Therefore, the ratioRn in the on state becomes smaller. However, by making the pitch oftrenches sandwiching the non-IGBT region than the pitch of trenchessandwiching IGBT forming region, the ratio of enlarged n⁺ emitter region(n⁺ accumulation region) with respect to ratio Rn becomes smaller, andhence approximately the same ratio Rn as in Embodiment 19 can beobtained.

Meanwhile, at a portion where gate electrode layer 117 extends over thefirst main surface, interlayer insulating film 119 on gate electrodelayer becomes thinner. Therefore, the larger the portion of gateelectrode layer 117 extending over the first main surface, the morelikely defective breakdown voltage between gate electrode layer 117 andemitter region 121, degrading production yield. Therefore, in view ofproduction yield, it is desirable that gate electrode layer 117 does notextend over the non-IGBT region, and the portion extending over thefirst main surface should be as small as possible. Therefore, thepresent embodiment is more effective in industrial application ascompared with Embodiment 19.

[Embodiment 21]

Referring to FIG. 94, as compared with Embodiment 19 shown in FIG. 92,the present embodiment differs in that a p⁺ diverter structure 141 isprovided on the first main surface. Between p⁺ diverter region 141 andIGBT forming region, there are a plurality of non-IGBT regions.

The structure of the present embodiment is in line-symmetry with respectto lines R-R′ and U-U′ of FIG. 94. Therefore, the structure between thelines R-R′ and U-U′ may be regarded as a unit cell, or, alternatively, astructure between one R-R′ line and another R-R′ line may be regarded asa unit cell. Here, the latter structure, that is, the structure betweenone R-R′ line and another R-R′ line is regarded as the unit cell.Therefore, in a region sandwiched by p⁺ diverter region 141 and IGBTforming region, there are three non-IGBT regions, for example. In otherwords, there are four trenches 117 between p⁺ diverter region 141 andIGBT forming region.

As in Embodiment 19, the larger the number of non IGBT region between p⁺diverter region 141 and IGBT forming region, the closer to 1 the regionRn. However, though it depends to some extent on the pitch of trenchesand the depth of the trench, if the number of non-IGBT region between p⁺diverter region 141 and IGBT region exceeds the range of 2 to 4, theration Rn begins to saturate.

Further, the n⁺ emitter region (n⁺ accumulation region) extended in theon state is formed only at a close vicinity (in the range of about 100Å) of the interface between gate oxide film 115 and silicon substrate105, which is the n⁻ region. Therefore, if the extended n⁺ emitterregion (n⁺ accumulation region) becomes too long, the resistance ofaccumulation region becomes too large to neglect. Therefore, thepractical number of non-IGBT region sandwiched between p⁺ diverterregion 141 and IGBT region is at most 4. In other words, the number oftrenches 117 between p⁺ diverter region 141 and IGBT forming region isat most 5.

In the semiconductor device in accordance with the present embodiment,p⁺ diverter region 141 is provided to assist turn off function whenthere are a lange number of trenches between IGBT forming regions andthere are a large number of non-IGBT regions. A p⁺ diverter region 141has a function of transferring part of the main current at the time ofturn off from the IGBT structure portion.

Generally, at the time of turn off of the IGBT, first, the n channeldisappears at a gate negative bias state as described above, andfinally, hole current is extracted as collector current of pnptransistor, from p⁺ contact region 111. At this time, if n⁺ emitterregion is enlarged significantly by the MAE structure, the ratio of p⁺contact region 111 included in IGBT structure on the cathode side withrespect to the unit cell becomes smaller. Therefore, holes areconcentrated at p⁺ collector region 111 at the time of turn off.Therefore, holes are not entirely extracted out from p⁺ collector region111, making longer the turn-off time.

The p⁺ diverter region 141 is provided in order to increase the ratio ofp type region occupying the unit cell. More specifically, by theprovision of p⁺ diverter region 141, hole current is extracted ascollector current of pnp transistor not only from p⁺ collector region111 but also from p⁺ diverter region 141 at the time of turn off.Therefore, concentration of holes at the p⁺ collector region 111 can beprevented, and hence the problem of longer turn-off time can be solved.

Further, p⁺ diverter region 141 also has a function of reducingradiation of current at off state. Therefore, it is more effective toform p⁺ diverter region 141 at a portion relatively distant from IGBTforming region.

[Embodiment 22]

Referring to FIG. 95, the structure of the present embodiment differsfrom the structure of Embodiment 21 shown in FIG. 94 in that gateelectrode layer 117 does not extend over non-IGBT region.

Other structures are approximately the same as those of Embodiment 21.Therefore, corresponding portions are denoted by the same referencecharacters and description thereof is not repeated.

In the semiconductor device in accordance with the present embodiment,as compared with Embodiment 21, the gate electrode layer 117 does notextend over non-IGBT region. Therefore, in the on state, there is not anenlarged n⁺ emitter region (n⁺ accumulation region), and hence ratio Rnin the on state becomes smaller. However, by making smaller the pitch oftrains sandwiching the non-IGBT region than the pitch of trenchessandwiching IGBT forming region, the ratio of enlarged n⁺ emitter region(n⁺ accumulation region) occupying the ratio Rn becomes smaller, andhence approximately the same ratio Rn as in Embodiment 21 can beobtained.

Meanwhile, at a portion where gate electrode layer 117 extends over thefirst main surface, interlayer insulating film 119 becomes thinner.Therefore, if there is gate electrode layer 117 extending over thenon-IGBT region and the ratio of gate electrode layer 117 extending overthe first main surface is large, defective breakdown voltage is likelybetween gate electrode layer 117 and emitter electrode 121, resulting indecreased production yield. Therefore, in view of the production yield,it is preferable that the portion of gate electrode layer 117 coveringthe first main surface is as small as possible. Therefore, the structureof the present embodiment is more effective in industrial application ascompared with the structure of Embodiment 21.

In Embodiments 11 to 22 described above, if the ratio of n⁺ emitterregion 109 is increased, the ratio Rn can be increased, as alreadydescribed with reference to FIG. 22. Therefore, the on-state voltage Vfat on-state can be reduced. Meanwhile, by increasing the ratio of p⁺contact region 111, the tail current at the time of turn off can bereduced, and hence turn off loss Eoff can be reduced.

In Embodiments 11 to 22 above, the width of n⁺ emitter region 109 isformed to be approximately the same as the width of p contact region111. However, the n⁺ emitter region 109 and p⁺ contact region 111 mayhave different widths in accordance with the requirement of on-statevoltage Vf and turn off loss Eoff.

Further, in Embodiments 11 to 22, n⁺ emitter region 109 and p⁺ contactregion 111 are arranged linearly and alternately. However, as alreadydescribed with reference to FIGS. 54 to 56, these may be arrangedconcentrically. When p⁺ contact region 111 is appropriately arrangedconcentrically, it becomes possible to extract minority carriers withhigh uniformity, and hence more quick and stable turn off becomespossible.

In all the embodiments above, the conductivity types, that is, p an ntypes may be reversed.

In all the embodiments above, n type buffer region 3 and 103 are formed.However, dependent on the rate or desired function of the device, n typebuffer region 3 or 103 may be omitted. Further, by changing thicknessand impurity concentration of n type buffer region 3, 103, necessarymain breakdown voltage, on switching property or the like of the devicecan be obtained.

Further, in the embodiment described above, an example in which entiresurface of p⁺ collector region 1, 101 is in contact with anode electrode19, 123 has been described. However, an n type high concentration regionmay be electrically connected so as to cause short circuit of a portionof semiconductor substrate 5 or n⁻ region 105 with a portion of anodeelectrode 19, 123. As the n type region is connected to anode region 19and 123, electrical characteristic of the diode can be varied.

Though the cross sectional shape at the bottom of trench 9 is flat inEmbodiments 1 to 9, the cross sectional shape of the bottom of thetrench may be rounded, as shown in Embodiments 11 to 14. Alternatively,the cross sectional shape of the bottom of trench 113 or the like shownin Embodiments 11 to 22 may be flat as shown in Embodiments 1 to 10.

In Embodiments 1 to 10 also, semiconductor device superior in on-statevoltage Vf can be obtained by making the depth of trench 9 to be withinthe range of 5 μm to 15 μm as in Embodiments 11 to 14.

In each embodiment, if the depth of trench 9 or 113 is at least 10 μm,the on-state voltage Vf can further be reduced.

For all the embodiments described above, gate electrode layers 13 and117 are electrically connected to each other at a region not shown.

In each embodiment, gate electrode layer 13, 117 is formed to projectupward from the first main surface (cathode surface) of thesemiconductor substrate.

This facilitates control of etching to form the gate electrode layer,and it also ensures stable device operation. This point will bedescribed in greater details in the following.

In the device structure shown in FIGS. 100 to 102, gate electrode layer507 is filled in trench 505. In this case, gate electrode layer 507 iscompleted by once forming conductive layer entirely over the first mainsurface of the semiconductor substrate to fill the trench 505 and byperforming etch back on the entire surface of the conductive layer.However, if the amount of etching is excessive, gate electrode layer 507comes not to oppose to a part of or whole n type turn off channel layer508. In such a case, channel is not generated at n type turn off channellayer 508 even when a voltage is applied to gate electrode layer 507,and hence device does not operate.

Meanwhile, in each embodiment, gate electrode 13, 117 have only to beformed to project upward from the first main surface of thesemiconductor substrate. This facilitates control of etching. In thiscase, gate electrode layer 13, 117 completely fills the trench.Therefore, instable operation caused by insufficient generation of thechannel can be prevented.

The semiconductor device in the first aspect of the present invention isa device of voltage controlled type in which control electrode layer isarranged opposing to a first impurity region and a low impurityconcentration region of the semiconductor substrate with an insulatingfilm interposed. Therefore, as compared with the conventional currentcontrol type device, the gate control circuit can be simplified.

Further, the device including a diode structure in accordance with thepresent invention is a bipolar device, and hence it has low steady loss.

Further, gate electrode layer provides n⁺ accumulation layer when apositive bias is applied, so that defective cathode area is increased,and hence on-state voltage Vf of the diode can be reduced.

Further, only the first impurity region is formed at the first mainsurface of the semiconductor substrate between trenches, good oncharacteristic can be obtained.

Preferably, in the above described aspect, a third impurity regionhaving different conductivity type from the first impurity region isformed at the first main surface of the semiconductor substrate,adjacent to the first impurity region with a trench interposed. Thisincludes turn off speed, reduces turn off loss, and improves switchingtolerance and short-circuit tolerance.

By adjusting the ratio of existence of the first and third impurityregions, desired turn off speed and desired on-state voltage Vf can beselected.

In the semiconductor device in accordance with another aspect of thepresent invention, the gate control type is of voltage controlled typeas already described with reference to the first aspect above.Therefore, gate control circuit can be simplified.

Further, since the device is a bipolar device, low steady loss isobtained.

As already described with reference to the first aspect above, an n⁺inversion layer can be formed in a p type region and n⁺ accumulationlayer can be formed in the n⁻ region by applying the positive bias tothe control electrode layer. Therefore, the effective cathode area isincreased and on-state voltage Vf of the diode can be reduced.

Further, at the main surface of the semiconductor substrate, a fourthimpurity region of a different conductivity type from the first impurityregion is provided adjacent to the first impurity region with a trenchinterposed. Therefore, turn off speed can be improved and turn off losscan be reduced.

By adjusting the ratio of existence of the first impurity region and thefourth impurity region, desired turn off speed and desired on-statevoltage can be selected.

In the semiconductor device in accordance with a still further aspect,the gate control is of voltage controlled type. Therefore, gate controlcircuit can be simplified.

Further, since the device is bipolar device, low steady loss can beobtained.

Further, as already described, it is possible to increase effectivecathode area by the gate potential to reduce the on-state voltage of thediode.

Further, the third impurity region together with the first impurityregion are regarded as effective cathode region. Therefore, cathode areaat on-state can further be increased, and on-state voltage of the diodecan further be reduced.

In the above described aspect, preferably, an isolating impurity regionis provided to surround the diode or thyristor forming region.Therefore, the capability of electrically isolating the diode or thethyristor from other regions can be improved, and breakdown voltage ofthe device and stability of the device can be improved.

In the above described aspect, since the depth of the trench from thefirst main surface is from 5 μm to 15 μm, the on-state voltage vf canfurther be reduced, and the trench can be readily made by presently usedapparatus.

In the semiconductor device in accordance with a still further aspect ofthe present invention, the ratio Rn is as high as 0.4 to 1.0. Therefore,efficiency in injecting electrons on the cathode side is improved ascompared with the prior art, and on-state voltage Vf can be reduced.

In the above described aspect, the depth of the trench is preferablyfrom 5 μm to 15 μm, on-state voltage Vf can further be reduced, and thetrench can be readily made by presently used apparatuses.

In the above described aspect, preferably, the conductive layer iselectrically connected to the control electrode layer, and the controlelectrode layer opposes to the region of the semiconductor substratesurface between the second and third trenches. Therefore, it becomespossible to increase the effective cathode area, and hence on-statevoltage of the diode can further be reduced.

In the above described aspect, preferably, the second ion impurityregion of lower concentration is formed at the semiconductor substratesurface between the second and third trenches. Therefore, thyristoroperation occurs when the device operates, and hence on voltage is lowerwhen a rated current is conducted.

In the above described aspect, preferably, the fourth impurity regionformed below the first impurity region has lower concentration than thefirst impurity region. Therefore, when a negative voltage is applied tothe control electrode layer at the time at off-state, a p⁺ inversionlayer is formed along the side walls of the trench, facilitatingextraction of holes. Therefore, switching characteristics, switchingwithstanding amount and short circuit withstanding amount can beimproved.

In the semiconductor device in accordance with a still further aspect ofthe present invention, the ratio Rn can be approximated by dimensions ofvarious portions. Further, since the approximated ratio Rn can be madeto 0.4 or higher, efficiency in injection of electrons on the cathodeside can be improved from the prior art example, and on-state voltage Vfcan be reduced.

In the method of manufacturing a semiconductor device in accordance witha present invention, at the semiconductor substrate between the secondand third trenches, only a low concentration region of the semiconductorsubstrate is positioned, and the first impurity region is not formed.Therefore, the object of improving device characteristic by increasingthe ratio Rn can be attained, and main breakdown voltage can bemaintained.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A semiconductor device in which current flows between first andsecond main surfaces of an intrinsic or a first conductivity typesemiconductor substrate, comprising: a first impurity region of a secondconductivity type formed on said first main surface side of saidsemiconductor substrate; and a second impurity region of the secondconductivity type formed on said second main surface of saidsemiconductor substrate, sandwiching, with said first impurity region, alow concentration region of said semiconductor substrate; wherein saidsemiconductor substrate includes a trench reaching said lowconcentration region of said semiconductor substrate from said firstmain surface through said first impurity region; said device furthercomprising: a third impurity region of the first conductivity type onsaid first impurity region to be in contact with a sidewall of saidtrench at said first main surface of said semiconductor substrate; afourth impurity region of the second conductivity type having a higherconcentration than said first impurity region, formed on said firstimpurity region and adjacent to said third impurity region at said firstmain surface of said semiconductor substrate; a control electrode layerformed in said trench to oppose to said first and third impurity regionsand said low concentration region of said semiconductor substrate withan insulating film interposed, for controlling current flowing betweensaid first and second main surfaces by an applied control voltage; afirst electrode layer formed on said first main surface of saidsemiconductor substrate and electrically connected to said third andfourth impurity regions; and a second electrode layer formed on saidsecond main surface of said semiconductor substrate and electricallyconnected to said second impurity region; wherein the followingexpression is satisfied where Dt represents depth of said trench fromsaid first main surface, Wt represents width of said trench, Derepresents depth of said third impurity region from said first mainsurface, We represents width of said third impurity region from one ofsaid trenches to another of said trenches, and Pt represents pitch ofadjacent said trenches:$\frac{{2\quad\left( {{We} + {Dt} - {De}} \right)} + {Wt}}{{2\left( {{We} + {Dt} - {De}} \right)} + {Pt}} \geqq {0.4.}$